According to Leviticus 23:1-4, the Sabbath is a feast:
Leviticus 23:1 And the LORD spake unto Moses, saying,
Leviticus 23:2 Speak unto the children of Israel, and say unto them, Concerning the feasts of the LORD, which ye shall proclaim to be holy convocations, even these are my feasts.
Leviticus 23:3 Six days shall work be done: but the seventh day is the sabbath of rest, an holy convocation; ye shall do no work therein: it is the sabbath of the LORD in all your dwellings.
Leviticus 23:4 These are the feasts of the LORD, even holy convocations, which ye shall proclaim in their seasons.
A "holy convocation" is religious festival during which no work was done.
How come you don't keep those "other" seven feasts listed in Leviticus 23?
The Ten Commandments are only a part of the OT Law...you can't just "pull out" the Ten Commandments and the Sabbath and ignore the "other" parts of the OT law. It is ONE law and it is indivisible.