Leviticus 23:21

And ye shall proclaim on the selfsame day, that it may be an holy convocation unto you: ye shall do no servile work therein: it shall be a statute for ever in all your dwellings throughout your generations.

Cross-reference

Leviticus 23:2 introduces 'sacred assemblies' as the Lord's appointed festivals—the same term used here for this day's proclamation.

Leviticus 23:4 repeats the command to proclaim sacred assemblies at appointed times, linking directly to the assembly here.

Exodus 12:16 similarly commands a sacred assembly with no work on the first and seventh days of Unleavened Bread—parallel to this feast day.

In Numbers 28:25, the same wording 'holy convocation, no ordinary work' appears for the seventh day of Unleavened Bread, reinforcing the pattern.