John Gill Commentary Numbers 28:18

"In the first day shall be a holy convocation: ye shall do no servile work;" — Numbers 28:18 (ASV)

In the first day shall be an holy convocation
The first of the seven days, which was kept in a very religious manner:

you shall do no manner of servile work therein ;
except by preparing food to eat; see (Exodus 12:16) .