礼拜(lǐ bài) - week; religious service, worship in Mandarin

礼拜 (lǐ bài) does two jobs. As a noun it is a week, interchangeable with 星期 in the counting frames: 下个礼拜, 三个礼拜, 礼拜三. In the phrase 做礼拜 it keeps its original meaning, going to a church service.

Of the three words for week, 星期 is the neutral standard, 周 is the written and slightly official one (上周, 周末), and 礼拜 is the colloquial one. It is especially common in southern Mandarin and in speech everywhere. Nobody will misunderstand you, but you would not put it in a business email.

Two things catch English speakers. First, the numbering starts on Monday, so 礼拜三 is Wednesday, not Tuesday, and Sunday breaks the pattern as 礼拜天 or 礼拜日 rather than 礼拜七. Second, 礼拜 needs a measure word when you are counting a span of time (三个礼拜) but not when you are naming a day (礼拜三). Getting 个 in the wrong slot is the usual slip.