A few Bible controversies never seem to die, some of them revolve around what day something happened. Here are four such controversies. My goal is to explain my understanding of each one.
- What day was Jesus Christ crucified?
- What day did Israel leave Egypt?
- What day is Pentecost (Shavuot)?
- What day did Israel cross the Red Sea?
Between The Ears BLOG INDEX, with titles and summaries.
What day was Jesus Christ crucified?
Most professing Christians believe Jesus was crucified on a Friday, and was resurrected on a Sunday, as in Good Friday to Easter Sunday. The problem is that Jesus said He would be in the heart of the earth (dead) for three days and three nights. And this would be the sign that He was the Messiah.
Mt 12:39 And answering He said to them, “An evil and adulterous generation seeks for a sign, and no sign will be given to it, except the sign of Jonah the prophet. 40 For just as Jonah was in the belly of the great fish three days and three nights, so the Son of Man will be in the heart of the earth three days and three nights.
Mk 15:42 And evening having arrived already, since it was the Preparation, that is, the day before Sabbath,Jn 19:31 Therefore the Jews, because it was the Preparation, so that bodies would not remain on the cross on the Sabbath--for that Sabbath was a high day--asked Pilate that their legs might be broken and they might be taken away.
It follows, therefore, that the Lord was crucified on our Wednesday; was buried on that day before sunset; and remained "three days and three nights' in the tomb, as foretold by Him in Matt. 12:40; rising from the dead on the third day', 'the first day of the week'. The fixed days and dates, at either end, hold the whole period as in a vice, and place the whole subject on a sure foundation. Companion Bible, Appendix 156 
What Day Did Israel Leave Egypt?
It is straightforward to determine the day of the Exodus, there are a couple clues that make it possible. Those clues are in the story of the giving of manna, explained in Exodus 16. Arriving in the Wilderness of Sin, the Israelites complained. God heard their complaint and promised them meat that night and bread (manna) in the morning. They received manna for six straight days then on the Sabbath, no manna. In order to get six days of manna in a row, it had to start on Sunday morning. Therefore, the day they arrived was a Sabbath. which was the 15th day of the second month.
Ex 16:1 On the fifteenth day of the second month after they had left the land of Egypt, the whole congregation of Israel set out from Elim and came to the Desert of Sin, which is between Elim and Sinai. 2 And there in the desert they all grumbled against Moses and Aaron.Ex 16:8 And Moses added, “The LORD will give you meat to eat this evening and bread to fill you in the morning, for He has heard your grumbling against Him. Who are we? Your grumblings are not against us but against the LORD.”
Chabad.org(Hebrew months) . Josephus writes that the food they brought from Egypt lasted 30 days, implying Iyar (second month) 15 and a 30 day Nisan - Antiquities Of the Jews , line 4401. Working back shows Passover on Tuesday night & Wednesday daytime, and the Exodus on Wednesday night & Thursday daytime. Hebrew days start at sunset.
Su Mo Tu We Th Fr Sa
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17 Passover the 14th, Exodus the 15th
18 19 20 21 22 23 24
25 26 27 28 29 30
2 3 4 5 6 7 8
9 10 11 12 13 14 15 Arrive in Wilderness of Sin
16 17 18 19 20 21 22 First week of manna
23 24 25 26 27 28 29
What day is Pentecost (Shavuot)?
Lev 23:15 You shall also count
for yourselves from the day after the Sabbath , from the day when you brought in the sheaf of the wave offering ; there shall be seven complete Sabbaths
Lev 23:16 You shall count fifty days to the day after the seventh Sabbath; then you shall present a new grain offering to the LORD.
Are there any other clues in the Bible? Yes. Note that the count starts on the day of the Wave Sheaf Offering (verse 15). Note also in verse 14 that the Israelites were prohibited from eating parched grain until the Wave Sheaf Offering.
Lev 23:14 Until this very day , until you have brought in the offering of your God , you shall eat neither bread nor roasted grain nor new produce Josh 5:11 Then on the day after the Passover, on that very day, they ate some of the produce of the land, unleavened cakes and roasted grain.
- They crossed on the Last Day of Unleavened Bread
- They crossed on the Fourth Day of Unleavened Bread
- They crossed after Unleavened Bread
For 8 days Israel camped at the Red Sea crossing point (days 17-24) while Pharaoh's army pursued them with 600 choice horse-drawn chariots - Exodus Route 
Last Day of Unleavened Bread Theory
Many teach that the Red Sea crossing happened on the seventh (last) day of Unleavened Bread. This includes branches of the Churches of God (other churches don't seem to comment on it), Jews, Messianic Jews, and individuals. Some quotes.
Jewish tradition says they crossed the sea on the seventh day of the Passover week - Chabad.org 
Seven days after Israel left Egypt after the first Passover, the new nation went through the Red Sea - Hallel.info 
The Israelites crossed the Red Sea on the last day of the Feast of Unleavened Bread - ucg.org 
“Speak to the children of Israel, that they turn and camp before Pi-hahiroth, between Migdol and the sea, opposite Baal Zephon; you shall camp before it by the sea.” This is the start of the 21st day—the last day of the Feast of Unleavened Bread - Don Roth .
Fourth Day of Unleavened Bread Theory
While most branches of the Churches of God teach the seventh day crossing, some offshoots of the Worldwide Church of God (WCG) teach that the crossing happened on the fourth day, which would be on the Sunday that we now call the Wave Sheaf Day or Pentecost. Before 1965, the WCG Correspondence Course "contained no assertions or conclusions about the Israelite's day-to-day passage during that initial seven-day Feast" - COG Eternal , HWA Library . In this theory, three days after the Wave Sheaf Day, Israel sacrificed in the wilderness on the seventh day of Unleavened Bread. Num 33:5-8 shows three encampments after Rameses: Succoth, Etham, and Pihahiroth. If each encampment lasts a day, the crossing was Sunday the fourth day. If Israel stayed at Etham for the weekly Sabbath, then travelled south on Sunday, they camped at two unidentified locations Sunday and Monday nights before reaching Pihahiroth, then they crossed on the seventh day. The sacrifice at Marah would have been three days after the Feast of Unleavened Bread. God has a pattern of timing important things on His Holy Days. While both theories have a big event happen on the seventh day (Red Sea crossing or sacrifice in the wilderness), the seventh day crossing theory neglects Wave Sheaf Day and has the sacrifice after the Feast of Unleavened Bread.
I see problems in both theories, fourth and seventh days. One problem in the seventh day theory is textual - it requires two unidentified camp sites that Moses didn't mention. Another problem is the sacrifice at Marah is after Unleavened Bread. One problem in the fourth day theory is logistics - getting a million+ people to walk that far that fast. Knowing the route of the Exodus might resolve the issue, but the scholars don't agree on that. So, like a number of things, do your own research and make up your own mind.