From the day after the Shabbat, the day you brought the sheaf of the wave offering [First Fruits], count off seven full weeks, until the day after the seventh week. Count off fifty days, and then present an offering of new grain to the LORD.
The Hebrew word Shavuot means "weeks". The above quotation from Leviticus 23 tells us to celebrate this day fifty days after a certain Shabbat. See the essay on First Fruits for a discussion about which day this was, which will in turn determine whether Shavuot may fall on any day of the week or whether it always falls on a Sunday.
Shavuot commemorates God's provision.
It celebrates the wheat harvest (similarly to how Pesach celebrated the barley harvest).
It also commemorates both the giving of the Covenant of Sinai and the giving of the New Covenant.
The new covenant was prophesied by Jeremiah and fulfilled by Yeshua:
"Here, the days are coming," says the LORD, "when I will make a new covenant with the house of Israel and with the house of Ya'akov (Jacob). It will not be like the covenant I made with their fathers on the day I took them by their hand and brought them our of the land of Egypt; because they, for their part, violated my covenant, even though I, for my part, was a husband to them," says the LORD.
"For this is the covenant I will make with the house of Israel after those days," says the LORD: "I will put by Torah within them and write it on their hearts; I will be their God, and they will be my people. No longer will any of them teach his fellow community member or his brother, 'Know the LORD'; for all will know me, from the least of them to the greatest; because I will forgive their sins and iniquity and remember these no more.
Centuries before Yeshua's ministry the Rabbis had decided that Shavuot was the anniversary of when God appeared upon Mount Sinai and instituted his covenant to the Jewish people.
However, the remembrance as bittersweet because at Sinai the Israelites had backed away from what God had wanted.
...when the people saw it they trembled. Standing at a distance they said to Moses, "You speak with us; and we will listen. But don't let God speak with us or we will die!"
Because of the Israelites' fear God changed them from a nation of priests (Exodus 19:5-6) to a nation with priests (Exodus 28:1). Traditional on the night when Shavuot began (remember that Jewish days start in the evening) was a vigil called Tikkun Lail Shavuot, when the Jewish people would plead with God, promising that if he would again offer to make Israel a people of priests that they would not refuse again.
The year that Yeshua died and rose God answered that vigil. The vigil is why in Acts 2:1 the believers were gathered in one place. The people of priests was finally established (First Peter 2:9). The physical presence of God upon Sinai in wind, sound, and fire was duplicated upon each of the believers present at that Pentecost.
Thus Shavout is the anniversary of when we received God's written word and his indwelling Spirit!
This cannot be definitely proved, since the Hebrew calendar is a lunar one, and months do not always have the same number of days from year to year. But the claim is very reasonable:
In the third month after the Israelites left Egypt—on that (zeh) day—they came to the Desert of Sinai. After they set out from Rephidim, they entered the Desert of Sinai, and Israel camped there in the desert in front of the mountain...
The phrase "on that day" is interpreted as the third day of the third month, Sivan. For most years this is 47 days after First Fruits. Then came three days of preparation:
And the LORD said to Moses, "Go to the people and consecrate them today and tomorrow. Have them wash their clothes. Set them apart to meet with me and be ready by the third day, because on that day the LORD will come down on Mount Sinai in the sight of all the people.
These additional three days bring the count of days from 47 up to 50.
Shavuot is a day about Adonai giving presents—especially those two covenants. Since this day celebrates them both it also emphasizes how they work together.
Most obviously the two covenants work together when the Holy Spirit writes God's teaching on our hearts.
But this is the covenant that I will make with the house of Israel after those days, says Adonai: I will put my teachings in their inward parts, and in their heart will I write it; and I will be their God, and they shall be my people...
He will teach you all things, and bring to your remembrance all things that I said to you.
The Spirit of Truth...will guide you into all truth.
Shavuot's most significant lesson may be that the written Word of God and the given Holy Spirit of God work together.
The written Word of God too easily leads to legalism without the Spirit, and the Spirit too easily leads to emotionalism without the written Word. Any believer can read, understand, and gain insight from scripture when guided by the Holy Spirit. It is a heresy called nicolaitism ("against the laity", see Revelation 2:6) to believe, as did the Pharisees of Mark 7:13 and some later Christian groups, that only people with special training or secret knowledge can comprehend scripture.
A common metaphor for God's written revelation is bread (i.e., Deuteronomy 8:3). A common metaphor for God's Spirit is water (i.e., John 7:38). Just as physical nourishment requires a balance of bread and water, our spiritual nourishment requires a balance of the written Word and the Spirit. Sadly, congregations exist that have at one point drifted too much to either extreme: emphasizing the Word or the Spirit to the exclusion of the other. The result is always spiritual malnourishment.
Recall from our essay about Pesach the four promises of Exodus 6:6-7 that we commemorated with four cups of wine at the Pesach Seder: the day the Israelites stood at the foot of Mount Sinai and heard the Ten Commandments is seen as the day the people were established as not just a family but also a nation: "And I will take you to me as a people." Similarly, on Pentecost Yeshua's followers were established as not just a Jewish sect but as citizens and ambassadors of the Kingdom of God. Being ambassadors of the Kingdom of God relates to our earlier Shavuot themes because, according to Acts 1:8, we receive the Holy Spirit in order to witness with power. We have the Spirit mainly to help us proclaim the witten Word! (Because God is generous we have it for other reasons as well, but its core purposes are to change us and empower us to change others. Thus there are more miracles where witnessing happens.)
Leviticus 17:8-9 teaches us that since the Temple is not standing we cannot perform animal sacrifices as Leviticus 23 commands.
We can still be thankful for God's provision. Shavuot, after all, was in part established to celebrate the wheat harvest.
Also, there are rabbinical stories about how God offered Torah to all the nations but only Israel accepted it. Just as Sukkot will be a day of praying for a "harvest" of people of all nations accepting God, Shavuot is traditionally a day of intercessory prayer for all the nations. We just read how Israel (and through it all of Yeshua's followers) had completed their years of repentance for backing away from fully accepting the covenant. Now we can pray for other nations that do not, as a nation, desire to follow God: that their years of waiting, before becoming a priesthood, be complete; that the "seeds are planted" for a a great "harvest" next Sukkot.
Other Shavuot traditions include: