The Feast of Firstfruits (The Feast of Weeks with the firstfruits) is observed on the day after the first Sabbath of the harvest season, which is Sunday in Christianity. This also symbolizes the resurrection of Jesus Christ, who is the "firstfruits." In Leviticus, it is only stated as the time of the firstfruits harvest, so a clear date is not specified. Therefore, Judaism generally observed the first Sabbath after Passover as the Feast of Firstfruits.
Exodus 34:22Three times in the year all your males shall appear before the Lord Yahweh, the God of Israel.
Leviticus 23:9“Speak to the children of Israel, and tell them, ‘When you have come into the land which I give to you, and shall reap its harvest, then you shall bring the sheaf of the first fruits of your harvest to the priest.Leviticus 23:10He shall wave the sheaf before Yahweh, to be accepted for you. On the next day after the Sabbath the priest shall wave it.Leviticus 23:11On the day when you wave the sheaf, you shall offer a male lamb without defect a year old for a burnt offering to Yahweh.