Chinese Vocabulary - Days in a Week 星期 and Month 月份

Days in a Week 星期 and Month 月份
The modern Chinese names for the days of the week are based on a simple numerical sequence. The word for 'week' is followed by a number indicating the day: 'Monday' is literally 'week one', 'Tuesday' is 'week two', etc. The exception is Sunday, where 天 tiān or 日 rì, both meaning 'day', are used instead of a number (日 rì is somewhat more formal than 天 tiān).
From http://www.cjvlang.com/Dow/dowchin.html