"Pentecost" is from the Greek word for “fiftieth [day].”
It was a festival in honor of God's giving of Torah (the Law) on Mt. Sinai, fifty days after the first Passover.
In Hebrew, the name of the feast is “Shavuoth,” meaning “weeks.” Fifty days is essentially a “week of weeks.” Seven weeks times seven days per week is forty-nine; but they add one day to that total, arriving at fifty, because count includes the starting day.
Notice that the same phenomenon of adding one for the starting day turns up in the French phrase for a week: “huit jours” (eight days!). That, in turn, led to a Beatle's song called “Eight days a week.”
The gift of the Holy Spirit was given to the 120 disciples in the Upper Room (Acts 2) on the Jewish feast of Pentecost (Shavuoth).
In the Jewish calendar, the count of the week-of-weeks begins the day after Passover. If Passover falls on a Tuesday in a particular year, Pentecost will fall on a Wednesday.
There are a few fixed points in the Scriptures:
Christians changed the starting point for counting fifty days from Passover to Easter Sunday. Our feast of the Fiftieth Day (Pentecost) always falls on a Sunday. It is a week of weeks after Easter, not a week of weeks after Passover.
This change in the way we calculate the Fiftieth Day introduces a discrepancy between the original Scriptural account (50 days after Passover, with day 1 being the day after Passover) and the liturgical system (50 days after Easter, with day 1 being Easter itself).
Scripturally, the “Church's first novena” (nine days of prayer while waiting for the gift of the Holy Spirit) counted the days from the Ascension to the Jewish festival of Pentecost (fifty days after Passover). Liturgically, there are now nine days “between” Ascension Thursday and Pentecost Sunday.
| DOW | Jewish count | Christian liturgy | Luke | John |
|---|---|---|---|---|
| Wed | -1 | Preparation Day | ||
| Thu | Passover | Last Supper = Passover | Last Supper NOT Passover! | |
| Fri | 1 | Good Friday = day after Passover | Preparation Day | |
| Sat | 2 | Passover on Saturday | ||
| Sun | 3 | day 1 | Resurrection on Sunday | Resurrection, Ascension, Gift of Spirit |
| Mon | 4 | |||
| Tue | 5 | |||
| Wed | 6 | |||
| Thu | End Passover week 7 | |||
| Fri | 8 | |||
| Sat | 9 | |||
| Sun | 10 | |||
| Mon | 11 | |||
| Tue | 12 | |||
| Wed | 13 | |||
| Thu | 14 | |||
| Fri | 15 | |||
| Sat | 16 | |||
| Sun | 17 | |||
| Mon | 18 | |||
| Tue | 19 | |||
| Wed | 20 | |||
| Thu | 21 | |||
| Fri | 22 | |||
| Sat | 23 | |||
| Sun | 24 | |||
| Mon | 25 | |||
| Tue | 26 | |||
| Wed | 27 | |||
| Thu | 28 | |||
| Fri | 29 | |||
| Sat | 30 | |||
| Sun | 31 | |||
| Mon | 32 | |||
| Tue | 33 | |||
| Wed | 34 | |||
| Thu | 35 | |||
| Fri | 36 | |||
| Sat | 37 | |||
| Sun | 38 | |||
| Mon | 39 | |||
| Tue | 40 | |||
| Wed | 41 | |||
| Thu | 42 | Ascension Thursday | Ascension (1) | |
| Fri | 43 | (1) | (2) | |
| Sat | 44 | (2) | (3) | |
| Sun | 45 | (3) | (4) | |
| Mon | 46 | (4) | (5) | |
| Tue | 47 | (5) | (6) | |
| Wed | 48 | (6) | (7) | |
| Thu | 49 | (7) | (8) | |
| Fri | Pentecost 50 | (8) | Gift of the Spirit (9) | |
| Sat | +1 | (9) | ||
| Sun | +2 | day 50: Pentecost | Jewish Pentecost |
Passover is dated by lunar cycles. It can fall on any day of the week. If we can rely on the tradition that Jesus died on a Friday, then John is identifying a different year from that specified in the synoptic gospels (Mt, Mk, Lk).
Pontius Pilate was procurator from 26 AD to 36 AD. That narrows things down somewhat; I think it has to be a historical fact that Jesus was crucified while Pontius Pilate was procurator of Judea. So that rules out one of my date calculations above.
This page gives a calculation of Passover dates from 26 AD to 34 AD. The author argues for Wednesday being the day of Passover. I'm inclined to disagree with that interpretation.
In the synoptics (Mt, Mk, Lk), Jesus dies the day AFTER Passover; in John, Jesus dies the day BEFORE passover. If our liturgical tradition has preserved the truth that Jesus died on a Friday, and if this fellow's calculations for the Passover dates are correct, then the synoptic year would be 34 AD and the Johannine year would be 33 AD.
A date of 33-34 AD seems nice to me–but it places a strain on Luke's estimate that Jesus was born in 4 BC, was “about 30” when He started His public ministry, that the public ministry lasted just a year or less (there is only one Passover in the synoptics), and that Jesus died the day AFTER the Passover meal. If we want to insist that Luke got the birth year right (in or before 4 BC) and that he is right about Jesus' age, then he has to be wrong about other aspects of the story.
I'm not too anxious about any of this. Our salvation comes from Jesus' priestly action of offering Himself as our Passover Lamb; we are saved by the blood of the Lamb, not by knowing what year He made this offering for us.