星期【xīngqī】

The days of the week are 星期 plus a number, counting from Monday.

星期一 Monday, 星期二 Tuesday, 星期三 Wednesday, 星期四 Thursday, 星期五 Friday, 星期六 Saturday.

Sunday breaks the pattern

Sunday is 星期天 in speech and 星期日 in writing. xīng qī星期qī七 does not exist.

周 and 礼拜

周一 to 周六 and 周日 mean the same thing and are shorter, common in writing and in schedules. 礼拜 is the colloquial version heard in the south.

No 是 needed

A date or a day takes no verb. 今天是星期三 is correct, but 今天星期三 is what people say.

The negative does need it: 今天不是星期三.

Last week, this week, next week

上个星期, 这个星期, 下个星期 – with 个, because 星期 takes a measure word.

Asking