The English word Sabbath comes from the Hebrew Shabbat.
Shabbat is a festival which occurs weekly. It begins when you can see three stars in the sky on Friday evening, and ends at the same time on Saturday evening.
Because of the way the days of creation are described in Genesis, "Evening came and morning came..." days are reckoned to begin in the evening at sunset.
Sometimes it is not possible to see the sky and count the stars, so the Shabbat times are calculated and published by Jewish organisations. Look at Chabad's site to see the times for the coming weeks.
Click here to find out what Jews count as 'work' on Shabbat.
In the Ten Commandments God commands Jews to:
"Remember to keep the sabbath day holy. You have six days to labour and do all your work. But the seventh day is a sabbath of the Lord your God; that day you shall not do any work... for in six days the Lord made heaven and earth, the sea and all that is in them, and on the seventh day he rested. Therefore the Lord blessed the sabbath day and declared it holy."