"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. He shall wave the sheaf before YHWH, to be accepted for you. On the next day after the Shabbat the priest shall wave it." Vayikra 23:10–11 · KWS
The Torah Mitzvah
The day after the Shabbat that falls during Unleavened Bread.
Firstfruits is timed off Shabbat. During the seven days of Unleavened Bread, on the day immediately following the weekly Shabbat, the first sheaf of the new barley harvest is brought to the priest and waved before YHWH. It is the firstfruits — a promise that the rest of the harvest will follow.
"But now Mashiach has been raised from the dead. He became the first fruit of those who are asleep." Korintim Alef 15:20 · KWS
The Scripture Observance
Acknowledge the Giver of the harvest.
Bringing the first of the harvest before YHWH is the practical confession that the harvest is His, not yours. The first cut is dedicated to Him. What follows in the field — and what follows in any provision He gives — flows from acknowledging the source.
"He is not here, for he has risen, just like he said." Mattityahu 28:6 · KWS
Yeshua in Firstfruits
He rose on the day He appointed for the first sheaf.
The women came to the tomb at the end of the Shabbat and found it already empty Mattityahu 28:1 — the resurrection had already happened, on Yom HaBikkurim, the day appointed for the first sheaf. The timing is argued in full on Shabbat.
Sha'ul names it directly: "Mashiach has been raised from the dead. He became the first fruits of those who are asleep" Korintim Alef 15:20 Not the metaphor of firstfruits — the moed of Firstfruits. The harvest the wave-sheaf promised is the resurrection of all who are His. He went first. The full harvest follows.
"Easter" is not in the Scriptures. The day He rose is.
What the Institutional Church Did
Untethered the resurrection. Renamed the day. Lost the moed.
Easter — untethered from YHWH's calendar at Nicaea and renamed for a Germanic dawn goddess (Ēastre) — severed the resurrection from the very moed YHWH appointed for it. The full account is on Pesach.