These verses establish when first fruits is:
Deuteronomy 26:1-4, 9-10
1 “And it shall be, when you come into the land which Yahuwah your Elohim is giving you as an inheritance, and you possess it and dwell in it,
2 that you shall take some of the first of all the produce of the ground, which you shall bring from your land that Yahuwah your Elohim is giving you, and put it in a basket and go to the place where Yahuwah your Elohim chooses to make His name abide.
3 And you shall go to the one who is priest in those days, and say to him, ‘I declare today to Yahuwah your Elohim that I have come to the country which Yahuwah swore to our fathers to give us.’
9 He has brought us to this place and has given us this land, “a land flowing with milk and honey”;
10 and now, behold, I have brought the first fruits of the land which you, O Yahuwah have given me.’
So what day was this?
10 Now the children of Israel camped in Gilgal, and kept the Passover on the fourteenth day of the month at twilight on the plains of Jericho.
11 And they ate of the produce of the land on the day after the Passover (fifteenth day – A Sabbath), unleavened bread and parched grain, on the very same day.
12 Then the manna ceased on the day after they had eaten the produce of the land (the sixteenth); and the children of Israel no longer had manna, but they ate the fruit of the land of Canaan that year.
And how is it related to Yahushua’s resurrection?
20 But now is Yahushua risen from the dead, and become the first fruits of them that slept.
21 For since by man came death, by man came also the resurrection of the dead.
22 For as in Adam all die, even so in Yahshua shall all be made alive.
23 But every man in his own order: Yahushua the firstfruits; afterward they that are Yahushua’s at his coming.
And this is reflected in Leviticus 23
9 Then Yahuwah spoke to Moses, saying,
10 “Speak to the children of Israel and say to them, ‘When you enter the land which I am giving you and reap its harvest, you shall bring the sheaf of the first fruits of your harvest to the priest.
11 He shall wave the sheaf before Yahuwah so that you may be accepted; the priest shall wave it on the day after the Sabbath.
12 Now on the day when you wave the sheaf you shall offer a male lamb one year old without blemish as a burnt offering to Yahuwah.
And which day was the day after the Sabbath? He was crucified upon the 14th. The 15th He and the people rested.
56 Then they went back and prepared spices and ointments and sweet-smelling herbs. And on the Sabbath they rested in accordance with the commandment [forbidding work].
The 16th He rose and became the first fruits.