A sabbath is a day reserved for rest or prayer, usually each week. The word Sabbath comes from the Hebrew word Shabbat (שַׁבָּת). This word is built from the Hebrew root Sh-B-T, meaning rest.
In the Hebrew Bible, or Tanakh, the root Sh-B-T is used to describe God resting on the seventh day. This is the day after He finished creating the universe. In the Bible, the idea of Shabbat, or Sabbath, as a holy day of rest starts there.
Remembering and keeping Sabbath is also one of the Ten Commandments. From that, the Abrahamic religions (Judaism, Christianity and Islam) all have a tradition of a holy day of rest or prayer set aside every week. These days are now very different from each other, including which day of the week they fall on.