What is the basis for the idiomatic understanding of "three days in the tomb" as two nights and a day?

The original calendar of Moses starts in the moon/month of Abib[early spring moon]. Each New Moon, the Yomim[24 hour periods of time beginning at dusk, and lasting till the following dusk] are recalculated, or renewed, or recounted[Our present calendar is Roman/Gregorian and is solar/sun based; very little recalculations are required- every leap year add a day, and by by not making years divisible by 100 to be a leap year.] You will find every observed Sabbath rest in the scripture will occur the 8th, 15th, 22nd, or 29th. Realize when you read "Month" in scripture, it means Moonth/Moon. Passover will always occur the late afternoon of Abib 14; and proceed into the 15th[Full Moon {New Moon is yom 1, special and separate from all the rest of the 29 yomim of the Moon/Month(“In the time of the earliest prophets, the New Moon stood in the same line with another lunar observance, the Sabbath. Ezekiel, who curiously enough frequently dates his prophecies on the New Moon … describes the gate of the inner court of the -new- temple looking eastward as kept shut for the six working days, but opened on the Sabbath and the New Moon.” Scribner's Dictionary of the Bible -1898 edit., p. 521. Also: “… each lunar month was divided into four parts, corresponding to the four phases of the moon. The first week of each month began with the new moon, so that, as the lunar month was one or two days more than four periods ofseven days, these additional days -new moon days- were not reckoned at all.” Universal Jewish Encyclopedia, Vol.10, p. 482. Article "Week.").}]

Yom 2,3,4,5,6,7 were/are the six working yomim, while 8 was/is Sabbath. Yom 9, 10, 11, 12, 13, 14 were/are working yomim, while 15 was/is always Sabbath/Full Moon}]. The children of Israel left Egypt the full moon of Abib, Passover night. Messiah instructed his disciples to prepare the "Passover" meal, the night of the betrayal. This was the 14th evening at dusk[precise timing for every Passover since Moses], making His torture and death taking place during "Feast of Unleavened Bread"[Leviticus 23:7]/15th Abib[and Sabbath]. His body was removed from the "tree" before dusk. Late afternoon 15th, all of 16, all of 17 & most of 18 Abib His physical body lay in the tomb. 16 Abib being yom 1[of the week], 17 Abib being yom 2 of the week, 18 Abib being yom 3 of the week, 19 Abib being yom 4 of the week. So, the scripture could very easily have described Mary and the other females taking perfumes to His tomb the 4th yom of the week; BUT it does not.

These 3 Yomim [16,17,18]would normally be[during other months/moons] working yomim[not Sabbaths] but Abib has a special Feast combination within it's time-frame. It has Passover at end of 14th, the 15th[a Sabbath/feast of Unleavened Bread beginning], Feast of First-Fruits[Leviticus 23:11] the 16th[another and unique Sabbath]. How do these Feasts and Sabbaths all fit into the picture? We've got to look at the Calendar Change that took place.

The Priests, Levites, religious leaders in Jerusalem at this time in history were transitioning to a new calendar format[they no longer base their "Sabbath" according to moon cycle: “The New Moon is still, and the Sabbath originally was, dependent upon the lunar cycle.” Universal Jewish Encyclopedia, p. 410. “…the Hebrew Sabbathon … was celebrated at intervals of seven days, corresponding with changes in the moon’s phases...” Encyclopedia Biblica, 1899. p. 4180

The New Testament says that Mary, and the other girls showed up at the tomb "after Sabbath, during the First day of the week" Matthew 28:1;John 20:1. Obviously, this descriptor is utilizing the "New Hellenistic/Roman Calendar". That "First Day" is Sunday[Webster's Dictionary]. So, let's break it down, with the intricate intertwining of feasts, calendar changes, New Moons & Sabbaths.

Messiah followed the original Commands of the Torah, as laid out by Moses, and had Passover meal with His Disciples late afternoon 14 Abib. He was grabbed later by Temple guards just after the 15th had begun[probably 9pm{ish}, the guards carried torches for lighting]. He was tortured, and died late afternoon the 15th. His body had to be removed from the stake[John 19:31] before the Priests, Levites, religious leaders[and probably most of the Jews in Jerusalem] would observe "their Passover"[according to the new calendar]. So Joseph of Arimathea and Nicodemus placed His body in the tomb before the 16th of Abib.

We can interpolate that the 16th of Abib was the time the Priests, Levites, religious leaders celebrated The Feast of Unleavened Bread Sabbath. So, the 16th is the First 24hour period Messiah lay in the Tomb, and it was a Sabbath[Mary and others couldn't trek to the tomb carrying perfumes during an observed Sabbath for fear of retribution]

The following 24 hour period/yom was the 17th of Abib. This would naturally be considered[by the Priests, Levites, religious leaders] the Feast of First Fruits Sabbath[Mary and others couldn't trek to the tomb carrying perfumes during an observed Sabbath for fear of retribution].

The following 24 hour period/yom was the 18th of Abib. At this juncture, we need to consider that Messiah said He would spend 3[full] yom in the tomb[as did Jonah in the whale]. What event could possibly take place to allow for ANOTHER Sabbath? The weekly Sabbath, of course...because Mary came to the tomb "early the first yom of the week". 18 Abib, the priests, Levites and religious leaders observed their weekly Sabbath. So Mary came to the tomb 19 Abib, finding it empty. Of course it was. He'd departed the tomb late afternoon the 18th[which would actually be "Third day"-1Corinthians15:4]. Remember the 15th of each moon/month is Sabbath, 16th is first yom, 17th second yom, 18th is third'.

This whole calendar switching is what throws nearly everybody off. Yeah, according to the Roman Calendar, Mary found the tomb empty the "first day of the week", but scripture Calendar places her find the 3rd day.

An online acquaintance, who is a Calvinist, and a linguist posted on a defunct Christian Message board and blog known as "Theologica" made this argument that the phrase "days" and "nights" is an idiom for "continued action", especially when that action is somehow unusual. Here is what he said, in that original post that I archived.

"Xd+Xn phrase is used when continuous duration is intended, and typically implies an extraordinary situation (Ex. 24:18, 1 Sam. 30:12, 1 Kings 19:8, Job 2:13; Jonah 1:17, Matt. 4:2). Where the idiomatic nature comes in, is that the formula has an understood distributive property, i.e. Xd+Xn=X(d+n). In other words X days and X nights is understood as equivalent to X DAYS-even if technically only part of the last day has occurred. The part is counted as a whole. Consequently, even in cases when, strictly speaking, only x-1 nights have actually occurred, the idiom still gives you X days and X nights. If this is indeed the linguistic convention, as I suggest, the specification of X nights may or may not be "literally" accurate. This may appear odd, but what we don't find in the Bible is a structure such as "seven days and nights" (evidently each noun requires its own numeral). Nor do we see "seven days and six nights" (it predates travel agents, I suppose). Now the sample is not huge, but what we do have suggests the practice is to give each of the two nouns a number and for the numbers to match. What I am suggesting is that even when there is no final night, the convention still matched the numbers, and everybody would have understood this and accounted for it accordingly.

There is another solution. In Genesis 1, the sun was not created until day 4, meaning that the night and day of the first three days was not governed by the sun, but by God in a supernatural way, demonstrating his superiority over the sun, demoting it from the position of deity that it enjoyed in many religions.

In Exodus 10, God cursed the sun over Egypt and brought darkness at midday.

In Joshua 10:

12 On the day the Lord gave the Amorites over to the Israelites, Joshua spoke to the Lord in the presence of Israel:

“Sun, stand still over Gibeon, and moon, over the Valley of Aijalon.” 13 And the sun stood still and the moon stopped until the nation took vengeance on its enemies.

Isn’t this written in the Book of Jashar?

So the sun stopped in the middle of the sky and delayed its setting almost a full day.

In 2 Kings 20:

8 Hezekiah had asked Isaiah, “What is the sign that the Lord will heal me and that I will go up to the Lord’s temple on the third day?”

9 Isaiah said, “This is the sign to you from the Lord that He will do what He has promised: Should the shadow go ahead 10 steps or go back 10 steps?”

10 Then Hezekiah answered, “It’s easy for the shadow to lengthen 10 steps. No, let the shadow go back 10 steps.” 11 So Isaiah the prophet called out to the Lord, and He brought the shadow back the 10 steps it had descended on Ahaz’s stairway.

All these examples show that God has the power over light and darkness and can even shorten or lengthen days. Pay particular note of the phrase "on the third day" in the quote above. If God determines when the days start and end, what can be said about the events of the passion week? In Mark 15:

33 At noon, darkness came over the whole land until three in the afternoon. 34And at three in the afternoon Jesus cried out in a loud voice, “Eloi, Eloi, lema sabachthani?” (which means “My God, my God, why have you forsaken me?”).

35 When some of those standing near heard this, they said, “Listen, he’s calling Elijah.”

36 Someone ran, filled a sponge with wine vinegar, put it on a staff, and offered it to Jesus to drink. “Now leave him alone. Let’s see if Elijah comes to take him down,” he said.

37 With a loud cry, Jesus breathed his last.

I believe that this miraculous event signals God's declaration of the end of one day and the beginning of another. It grew dark at noon, then before the normal sunset, light returned, then the normal sunset. Thus the three nights are:

1) The miraculous night at midday on Friday
2) The normal Friday night
3) The normal Saturday night

Clarification:

• Night #1 (Supernatural darkness):
  • Friday noon until a little past three pm (the ninth hour)
  • Jesus dies during this period of night and is carried to the tomb
  • A short night
• Day #1:
  • Friday after supernatural darkness ends thru sunset
  • A short day
• Night #2:
  • Friday sunset to Saturday sunrise
  • A whole night
• Day #2:
  • Saturday dawn to Saturday sunset
  • A whole day
• Night #3: Saturday sunset to Sunday sunrise
  • A whole night
• Day #3: Sunday sunrise to resurrection (shortly after sunrise)
  • A portion of a day

Thus all or parts of three days and all or poarts of three nights, but with one of the nights and days shortened supernatuurally.

Unfortunately I can't find the original source (yet), but according to this commentator, the Jerusalem Talmud states that a portion of a day was as a whole.

The Jerusalem Talmud quotes rabbi Eleazar ben Azariah, who lived around A.D. 100, as saying: “A day and night are an Onah [‘a portion of time’] and the portion of an Onah is as the whole of it” (from Jerusalem Talmud: Shabbath ix. 3, as quoted in Hoehner, 1974, pp. 248-249, bracketed comment in orig.).

The Jerusalem Talmud is also known as the Palestinian Talmud.

PS As the answer to the OP question, the reference (the argument amongst the early sages like Eleazar ben Azariah) is to the command "be ready on the 3rd day" (Ex. 19:15) with day defined earlier in Gen. 1:5, 8 (dark/evening and light/morning). So, some sages thought in terms of periods (dark or light) to make a day. When is the third day? Some thought at least 4 periods, some 5, some 6 (Ishmael). One said always 5 (Aqiva). Following are examples.

4 PERIODS

before sunset (1 period and day 1)

night, morning (2 periods (or 3 total) and day 2)

night (1 period (or 4 total) and day 3)

5 PERIODS

night, morning (2 periods and day 1)

night, morning (2 periods (4 total periods) and day 2)

after sunset (1 period (5 total) and day 3)

6 PERIODS

night, morning (2 periods and day 1)

night, morning (2 periods (4 total) and day 2)

night, morning (2 periods (6 total) and day 3)

So, traditional Christianity counts the sign of Jonah as 4 periods to mean 3 days (Fri-Sun).

Some count the full 6 periods (3 days and 3 nights) in ways like Thursday after sunset to Sabbath at sunset.

What is totally wrong is the newish theory of death on Wednesday to either resurrection at Saturday sunset (7 periods or 4 days) or Wednesday to Sunday resurrection (9 periods or 5 days).