礼拜 as 'week' is colloquial, especially popular in southern China and Taiwan. 星期 (xīngqī) is the standard/formal term. Both are perfectly understood everywhere.
usage
Days of the week: 礼拜一 through 礼拜六 (Mon-Sat), 礼拜天/礼拜日 (Sunday). Same pattern as 星期一, etc.