The Feast of Firstfruits included the presentation of firstfruits of the spring barley harvest in the Promised Land. The Israelites also offered a lamb, flour, and wine, all representative of God"s provisions of spiritual and physical food and drink for His people ( Leviticus 23:9-14). They presented this offering on the day after the Sabbath following Passover. The ancients regarded the firstfruits (Heb. shavuot) as a kind of down payment with more to follow.Jesus arose from the grave on this day as the firstfruits of those who sleep in death ( 1 Corinthians 15:20). [source][source][source]
In modern times it is customary for observant Jews to stay up the entire night of Shavuot studying and discussing the Torah. The tradition that the Israelites had fallen asleep the night before God gave them the Torah and Moses had to awaken them is the basis of this custom. [source][source][source]
"In order to acknowledge that the LORD provides the needs of their life, God"s people must present the first of their income to him as a token of their devotion." [1][source]