Sunday (// or //) is the day of the week between Saturday and Monday. For most Christians, Sunday is observed as a day for worship of God and rest, due to the belief that it is Lord's Day, the day of Christ's resurrection.
Sunday is a day of rest in most Western countries, part of 'the weekend'. In most Muslim countries, and Israel, Sunday is a working day.
According to the Hebrew calendars, traditional Christian calendars, Sunday is literally the "first day" of the week. According to the International Organization for Standardization ISO 8601 Sunday is the seventh and last day of the week.
No century in the Gregorian calendar starts on a Sunday, whether its first year is '00 or '01. The Jewish New Year never falls on a Sunday. (The rules of the Hebrew calendar are designed such that the first day of Rosh Hashanah will never occur on the first, fourth, or sixth day of the Jewish week; i.e., Sunday, Wednesday, or Friday).