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.

Ye shall proclaim on the selfsame day ... an holy convocation. Though it extended over a week, the first day only was held as a Sabbath, both for the national offering of first-fruits and a memorial of the giving of day only was held as a Sabbath, both for the national offering of first-fruits and a memorial of the giving of the law.