Could you please tell me which day is the Sabbath?
The Sabbath was a special day of rest under the Old Law. It referred to the seventh day, or last day, of the week. "Six days you shall labor and do all your work, but the seventh day is the Sabbath of the LORD your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates. For in six days the LORD made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the LORD blessed the Sabbath day and hallowed it" (Exodus 20:9-11). In America we call that day "Saturday."
Under the Old Law, certain holidays were also declared to be special Sabbaths, such as the Passover day and the Day of Atonement. The Sabbath rules applied to these special days (or some times called "holy convocations" of which the weekly Sabbath was one of them). The special Sabbaths did not always fall on a particular day of the week.
"Speak to the children of Israel, and say to them: 'The feasts of the LORD, which you shall proclaim to be holy convocations, these are My feasts. Six days shall work be done, but the seventh day is a Sabbath of solemn rest, a holy convocation. You shall do no work on it; it is the Sabbath of the LORD in all your dwellings. These are the feasts of the LORD, holy convocations which you shall proclaim at their appointed times. On the fourteenth day of the first month at twilight is the LORD'S Passover. And on the fifteenth day of the same month is the Feast of Unleavened Bread to the LORD; seven days you must eat unleavened bread. On the first day you shall have a holy convocation; you shall do no customary work on it. But you shall offer an offering made by fire to the LORD for seven days. The seventh day shall be a holy convocation; you shall do no customary work on it'" (Leviticus 23:2-8).