Rashi on Numbers 28:26/רש"י על במדבר כ״ח:כ״ו

1וביום הבכורים AND ON THE DAY OF THE FIRSTLINGS — The Feast of Weeks is called the firstlings of the wheat harvest (Exodus 34:22) because of the two loaves (Leviticus 23:17) which were the first of the wheat meal offerings which were brought from the new grain (Menachot 84b).