Why is the Crucifixion believed to have happened on Friday?

Traditional. In the Bible, a part of a day is often counted as a day. Thus part of Friday is the first day, Saturday is the second day, and part of Sunday is the third day in the tomb. In this case, Jesus was in the tomb a little less than two full days, but parts of three calendar days.

SDA. An alternative hypothesis (often used by Seventh Day Adventists) is that the Thursday Passover was considered a sabbath. In that case the crucifixion was on Wednesday afternoon, Jesus was in the earth Thursday, Friday and Saturday, and rose from the dead Saturday night. He was not seen until Sunday, but was actually resurrected on Saturday. In this case, Jesus was in the tomb three full days.

Miraculous Night. For those who are bothered by the "three nights" count, add a night for the darkness that descended on Friday afternoon. This is a miraculous night. Night fell, Jesus died, then the light returned briefly, then normal Friday night came. This then lets us count a third night.

44 It was now about noon, and darkness came over the whole land until three in the afternoon, 45 for the sun stopped shining. And the curtain of the temple was torn in two. 46 Jesus called out with a loud voice, “Father, into your hands I commit my spirit.” When he had said this, he breathed his last. (Luke 23:44-46)

A 2008 paper by Iver Larsen gives support for the idea that partial days are counted as full days in the Bible:

It has now been generally accepted that the Bible uses inclusive counting of days, a system still used today in Africa and parts of Asia and South America. This is most clearly seen from Lev 23:15-16 which says (NIV):

”From the day after the Sabbath, the day you brought the sheaf of the wave offering, count off seven full weeks. Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the LORD.”

The only way 7 full weeks, 49 days, from the day after the Sabbath to the day after the 7th Sabbath, can be counted as 50 is when both the beginning and ending days are included in the counting. The system is confirmed by each and every reference to the counting of days in the Bible. Compare also Hos 6:2:

”After two days he will revive us; on the third day he will restore us.”

Another important aspect of Biblical counting is that part of a day was counted as a whole day. This is most clearly seen by the fact that Jesus was raised on the third day. The first day is Friday, even though Jesus was only dead for about 3 hours that day. However, the length of the portion of the day is irrelevant. That portion of the day is counted as the first day. The third day started at sunset on Saturday. We don’t know the exact time when Jesus was raised, but it was apparently early Sunday morning before sunrise. But it is still the third day.Expressions like ”x days and x nights” are fairly common in the Bible. Some examples are: Gen7:4, 7:12, Exo 24:18, 34:28, 1 Kings 19:8; 1 Sam 30:12, Jonah 2:1, Job 2:12. (This is not a full list.)

See https://www.academia.edu/1040871/Counting_of_days_and_nights_in_the_Bible for details.

Why is the Crucifixion believed to have happened on Friday?

The biblical evidence for a Wednesday or Thursday crucifixion is very weak.

The Crucifixion is believed to have happened on Friday because it is both traditional belief and “all four Gospels agree to within about a day that the crucifixion was at the time of Passover, and all four Gospels agree that Jesus died a few hours before the commencement of the Jewish Sabbath, i.e. he died before nightfall on a Friday.”

Astronomical analysis

All four Gospels agree to within about a day that the crucifixion was at the time of Passover, and all four Gospels agree that Jesus died a few hours before the commencement of the Jewish Sabbath, i.e. he died before nightfall on a Friday (Matt 27:62; 28:1; Mark 15:42; Luke 23:54; John 19:31, 42). In the official festival calendar of Judaea, as used by the priests of the temple, Passover time was specified precisely. The slaughtering of the lambs for Passover occurred between 3pm and 5pm on the 14th day of the Jewish month Nisan (corresponding to March/April). The Passover meal commenced at moonrise (necessarily a full moon) that evening, i.e., at the start of 15 Nisan (the Jewish day running from evening to evening) (Leviticus 23:5; Numbers 28:16). There is an apparent discrepancy of one day in the Gospel accounts of the crucifixion which has been the subject of considerable debate. In John's Gospel, it is stated that the day of Jesus' trial and execution was the day before Passover (John 18:28 and 19:14), Hence John places the crucifixion on 14 Nisan. Likewise the Apostle Paul, in his First Epistle to the Corinthians, implies Jesus died on a 14 Nisan ("sacrificed as a Passover lamb", 1 Cor 5:7), and was resurrected on the Jewish festival of the first fruits, i.e. on a 16 Nisan (1 Cor 15:20). The correct interpretation of the Synoptics is less clear. Thus some scholars believe that all 4 Gospels place the crucifixion on Friday, 14 Nisan, others believe that according to the Synoptics it occurred on Friday, 15 Nisan. The problem that then has to be solved is that of determining in which of the years of the reign of Pontius Pilate (AD 26–36) the 14th and 15th Nisan fell on a Friday. - Chronology of Jesus

Christian Tradition Tells us that Christ rose “on the third day” and “not after three days”. This is bared out in the following verses:

• ...and on the third day be raised. - Matthew 16:21

• ...and on the third day he will be raised. - Matthew 17:32

• ...and on the third day be raised. - Luke 9:22

• ...and on the third day he will rise again. - Luke 18:33

• ...but God raised him on the third day and allowed him to appear - Acts 10:40

• ...and that he was raised on the third day - 1 Corinthians 15:4

A Good Friday Crucifixion of Jesus is a traditional belief and dates well into the Early Church. The Early Church never held the Lord’s Crucifixion on any other day of the week. The theory that Christ died on a Wednesday is only about 300 years old, whereas the Good Friday tradition is nearly 2000 years old.

Good Friday, called Feria VI in Parasceve in the Roman Missal, he hagia kai megale paraskeue (the Holy and Great Friday) in the Greek Liturgy, Holy Friday in Romance Languages, Charfreitag (Sorrowful Friday) in German, is the English designation of Friday in Holy Week — that is, the Friday on which the Church keeps the anniversary of the Crucifixion of Jesus Christ.

Parasceve, the Latin equivalent of paraskeue, preparation (i.e. the preparation that was made on the sixth day for the Sabbath; see Mark 15:42), came by metonymy to signify the day on which the preparation was made; but while the Greeks retained this use of the word as applied to every Friday, the Latins confined its application to one Friday. Irenaeus and Tertullian speak of Good Friday as the day of the Pasch; but later writers distinguish between the Pascha staurosimon (the passage to death), and the Pascha anastasimon (the passage to life, i.e. the Resurrection). At present the word Pasch is used exclusively in the latter sense. The two Paschs are the oldest feasts in the calendar.

From the earliest times the Christians kept every Friday as a feast day; and the obvious reasons for those usages explain why Easter is the Sunday par excellence, and why the Friday which marks the anniversary of Christ's death came to be called the Great or the Holy or the Good Friday. The origin of the term Good is not clear. Some say it is from "God's Friday" (Gottes Freitag); others maintain that it is from the German Gute Freitag, and not specially English. Sometimes, too, the day was called Long Friday by the Anglo-Saxons; so today in Denmark. - Good Friday

To declare that Christ laid in the tomb for 72 hours is to not being able to understand Bible facts and to not understand about Jewish customs and idioms. Being in the tomb parts of three days, perhaps about 36 hours (which a Friday crucifixion-Sunday resurrection would allow) is enough to demonstrate that the Jewish custom of counting partial days as full days, is to be understood here.

The Angus-Green Cyclopedic Handbook of the Bible, page 351, says: “It is to be observed that the Jews and other Orientals generally speak of any part of a day, or a period of time as if it were the whole. In like manner, fractions of a day are in England treated as legally whole days.”

Was Jesus in the tomb for three days and three nights?

It is significant that “Every occurrence of the ‘the third day’ with reference to Jesus’ resurrection in the Gospels is put in the dat. (dative case) without an accompanying preposition” (Dan Wallace, Greek Grammar Beyond the Basic, Zondervan, p. 156). The significance of this is that nouns used in the dative case like “the third day” express a point in time rather than duration of time. So it means, “at a point in time, on the third day.”