That almost sounds like it could be a fifth question during the Passover Seder.
Surprisingly, or maybe not so much, there is disagreement on how to interpret the starting point – and thus the ending point – of the counting of the omer. The starting point is the day of the Omer, the Wave Sheaf, the day after the Sabbath.
He shall wave the sheaf before the LORD for you to be accepted; on the day after the Sabbath the priest shall wave it.
(Leviticus 23:11 NASB)
The disagreement is over which Sabbath is being referenced here. Traditional Judaism and most Messianic Jews understand it to be the day after the High Sabbath on the first day of the Feast of Unleavened Bread. That day is always 16 Aviv but could be any day of the week. Some other groups, including Karaite Jews, Christians and non-Jewish Sabbath keepers, believe it to be the day after the weekly Sabbath during the Feast of Unleavened Bread. That is not a specific day of the month, but is always Sunday. For Christians, the count begins on Easter.
Most Jewish translations of Leviticus 23:15-16 read something like this:
And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to the LORD.
(Leviticus 23:15-16 JPS Tanakh)
For traditional Judaism and most Messianic Jews who start counting on 16 Aviv, it always ends seven weeks later on 6 Sivan no matter what day of the week that is.
Most Christian translations of Leviticus 23:15-16 read something like this:
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. You shall count fifty days to the day after the seventh Sabbath; then you shall present a new grain offering to the LORD.
(Leviticus 23:15-16 NASB)
For Christians, Karaite Jews and others starting the count on Sunday, it always ends on Sunday seven weeks later. Thus Shavuot, or Pentecost, always falls on Sunday no matter what day of the month that is.
We are not taking a position on which method of counting, if either, is correct. Instead, we have provided a count that covers both methods and gives both Hebrew and Gregorian calendar dates for each day of the count. We encourage you to be obedient to the commandment and count by whatever method you believe to be appropriate. Be willing to fellowship with those who are counting differently without contention.
You can find a calendar for counting the omer beginning on 16 Aviv here.
You can find a calendar for counting the omer beginning on Sunday here.