Thursday (i// or //) is the fourth or fifth day of the week, betweenWednesday and Friday. According to the ISO 8601 international standard adopted in most western countries, it is the fourth day of the week. In countries that use the Sunday-firstconvention, Thursday is defined as the fifth day of the week. It is the fifth day of the week in the Judeo-Christian calendar as well, and was defined so in the ancient Mesopotamian and biblical calendars. It falls between Wednesday and Friday. The name is derived from Old English Þūnresdæg and Middle English Thuresday, which means "Thor's day".