A sabbath is a day reserved for rest or prayer each week. Abrahamic religions have different kinds of sabbaths.
Sabbath (as the verb shavath) is first mentioned in the creation story of Genesis. The seventh day is reserved as a day of rest and made holy by God. Observing and remembering the Sabbath is one of the Ten Commandments.
In Judaism, the Shabbat is a weekly day of rest. It lasts from sunset on Friday until the appearance of three stars in the sky on Saturday night.
The Christian Sabbath is usually observed on the Sunday rather than Saturday.
In Islam, the Sabbath is observed as Jumu'ah. This is a communal prayer that Muslims hold every Friday, just after noon.