星期又作周或者礼拜，是一个时间单位，每星期有7日。

中国古代有七曜这一词，首现于东晋范宁的《春秋谷梁传序》：“阴阳为之愆度，七曜为之盈缩。”杨士勋疏：“谓之七曜者，日月五星皆照天下，故谓之曜。”五星指火、水、木、金、土，但与后来的七曜无关，因为在唐代以前还未有使用七星指代轮回的7日或一周；古巴比伦人首先使用7日为一周的时间单位，后来犹太人把它传到古埃及，又由古埃及传到罗马，公元3世纪以后，就广泛地传播到欧洲各国。唐朝时期，在西域和中国的交流中传入了中土，其后再传入日本，到了民国以后被广泛采用。

A week is a time unit equal to seven days. It is the standard time period used for cycles of work days and rest days in most parts of the world, mostly alongside (but not strictly part of) the Gregorian calendar.

The days of the week were named in different languages after classical planets, various deities (example: Thursday – Thor's day, a variation after Jupiter's day from Roman times) and heavenly bodies (example: Sunday – Sun's day) and other sources. In English, the names are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday.