Tuesday is the day of the week between Monday and Wednesday. According to international standard ISO 8601, Monday is the first day of the week; thus, Tuesday is the second day of the week. According to many traditional calendars, however, Sunday is the first day of the week, so Tuesday is the third day of the week. In some Muslim countries, Saturday is the first day of the week and thus Tuesday is the fourth day of the week.

In many languages, the names given to the seven days of the week are derived from the names of the classical planets in Hellenistic astronomy, which were in turn named after contemporary deities, a system introduced by the Sumerians and later adopted by the Babylonians from whom the Roman Empire adopted the system during late antiquity. In some other languages, the days are named after corresponding deities of the regional culture, beginning either with Sunday or with Monday. The seven-day week was adopted in early Christianity from the Hebrew calendar, and gradually replaced the Roman internundinum.