After Passover, Unleavened Bread, and Firstfruits is the Feast of Weeks.
And you shall count for yourselves from the day after the Sabbath, from the day that you brought the sheaf of the wave offering: seven Sabbaths shall be completed.
Count fifty days to the day after the seventh Sabbath; then you shall offer a new grain offering to the LORD. Lev 23:15-16
From one of the Jewish websites I found this: According to the Torah (Lev. 23:15), the Hebrews were obligated to count the days from the second night of Passover to the day before Shavu’ot, seven full weeks.
This period is known as the Counting of the Omer.
An omer is a unit of measure.
On the second day of Passover, in the days of the Temple, an omer of barley was cut down and brought to the Temple as an offering.
Every night, from the second night of Passover to the night before Shavu’ot, we recite a blessing and state the count of the omer in both weeks and days. So on the 16th day, you would say, “Today is sixteen days, which is two weeks and two days of the Omer.”
The counting is intended to remind us of the link between Passover, which commemorates the Exodus, and Shavu’ot, which commemorates the giving of the Torah. It reminds us that the redemption from slavery was not complete until we received the Torah.
According to Jewish tradition, Moses received the Law at Shavu’ot, or the Feast of Weeks.
We know this day as Pentecost when something wonderful happened to the Christians in Jerusalem.
Jeremiah said, “This is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. No more shall every man teach his neighbor, and every man his brother, saying, ‘Know the LORD,’ for they all shall know Me, from the least of them to the greatest of them, says the LORD. For I will forgive their iniquity, and their sin I will remember no more.” Jer 31:33-34
Joel was quoted by Peter on the day of Pentecost; And it shall come to pass afterward
That I will pour out My Spirit on all flesh; Your sons and your daughters shall prophesy,
Your old men shall dream dreams, Your young men shall see visions. And also on My menservants and on My maidservants I will pour out My Spirit in those days. Joel 2:28-29
Notice that both have a reference to “those days”.
Jesus promised He would not leave us alone but send a Comforter; a Helper.
That was fulfilled 50 days after Passover on Pentecost, the Feast of Weeks.