John Gill Commentary Numbers 28:26

"Also in the day of the first-fruits, when ye offer a new meal-offering unto Jehovah in your [feast of] weeks, ye shall have a holy convocation; ye shall do no servile work;" — Numbers 28:26 (ASV)

Also in the day of the firstfruits
When the firstfruits of the wheat harvest were brought unto the Lord, which was the day of Pentecost, fifty days from the sheaf of the wave offering being brought:

when you bring a new meat offering unto the Lord ;
that is, a meat offering made of the new corn, which were two wave loaves of two tenth deals of fine flour, baked with leaven, (Leviticus 23:15–17)

after your weeks be out ;
the seven weeks from the passover to Pentecost, even seven complete sabbaths or weeks, (Leviticus 23:15)

you shall have an holy convocation, you shall do no servile work ;
see (Leviticus 23:21) .