Why do we celebrate Easter only 2 days after Good Friday?

There are two considerations to answer your question. One is what is a Jewish day? Two is does a part of a day count as a whole day?

The Jewish day (your comment about 1st century time keeping) is measured from sunset to sunset.

Now, assuming that "part of a day counts as a whole day", whatever happens before sunset on Friday is considered by some as day 1. At sunset Friday is the beginning of Saturday (Sabbath), which is day two. At sunset Saturday is the beginning of Sunday, which is day three.

With that in mind, most of Christianity believes Christ died and was buried on Friday before sunset (day 1). He was in the grave on Saturday (day 2). He rose out from the dead about sunrise (day 3). So, we get 3 days.

On the other hand, if someone believes that the "part day" does not mean the "whole day", then the Friday to Sunday period doesn't work.

Some groups assert that Christ died and was buried on Wednesday and resurrected Sunday. You can't ignore that in your count. Wed-day 1. Thur-day 2. Fri-day 3. Sat-day 4. Sun-day 5. This doesn't work either.

The confusion about the 3 days and 3 nights lies with the false idea that "in the heart of the earth" can only mean "in the tomb/grave". Instead, "heart of the earth" refers to this psalm.

For God is my King of old, working salvation in the midst [heart] of the earth. Ps 74:12

The word "midst" is "qereb", meaning inward, midst, heart, etc (Strong's).

When was that? As Christ said, from Passover sufferings to resurrection glory, the three days and three nights.

EDIT TO ADD:

To reinforce this whole period of sufferings to glory, rather than only from the tomb and out of the tomb, here are examples.

The sufferings of Our Lord, which culminated in His death upon the cross, seem to have been conceived of as one inseparable whole from a very early period. Even in the Acts of the Apostles (i, 3) St. Luke speaks of those to whom Christ "shewed himself alive after his passion" (meta to mathein autou). NewAdvent Passion

Tertullian uses the same passion measure from sufferings to glory.

In like manner does He also know the very time it behoved Him to suffer, since the law prefigures His passion.

Luke 23 after describing the Crucifixion tells us in verse 54

And that day was the Preparation and the Sabbath drew on

Luke 24, describing the day of Resurrection begins with

Now upon the first day of the week, very early in the morning.

So we have that the Crucifixion was on the day of preparation and the Resurrection on the first day of the week.

The Jewish Sabbath is Saturday and the day of Preparation for the Sabbath is Friday. The first day of the week is Sunday.

So it is primarily from the gospel accounts that we get the Friday and the Sunday as the dates of the Crucifixion and Resurrection.

Although much has been said about how, and even whether, this can be harmonised with references to the third day and three days and three nights, neither of those phrases is of primary importance in the days of Good Friday and Easer Day. They come directly from the Gospel, and the common interpretation of the day of preparation of the Sabbath as meaning Friday, and the first day of the week as meaning Sunday.

Now what is the difference then this Sabbath day than any other Sabbath day (Fri evening to Sat Evening?) For that Sabbath was a High Holy Day.

John 19:31

Since it was the day of Preparation [for the Sabbath], in order to prevent the bodies from hanging on the cross on the Sabbath (for that Sabbath was a High Holy Day) the Jews asked Pilate to have their legs broken to hasten death and the bodies taken away.

Such as the Feast of Tabernacles has two High Holy Days, the First day is a high holy day and the 8th day was also a High Holy Day, those High Holy Days are also called a Sabbath even though it was on another day of the week not being on God's 7 day, (Friday evening-Saturday evening) So, since Jesus was killed at evening and he was to be dead for 3 days and 3 nights, he would have to rise up at evening 72 hours later which was Saturday night at Evening and when they went to his tomb Early Sunday morning, he had ALREADY risen. which was the evening before. That high Holy Day was the first day of Unleavened bread which that year was Wednesday Evening. Remember that the only sign that Jesus is our Messiah:

Matthew 12:39

But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: 40 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.