礼拜天 and 星期天 (xīngqītiān) both mean Sunday. 礼拜 originally means 'worship' (from Christian church services), so 礼拜天 literally means 'worship day.' More colloquial than 星期天 in many regions.
mistakes
The days Monday through Saturday use numbers (礼拜一 through 礼拜六), but Sunday breaks the pattern — it's 礼拜天 or 礼拜日, not 礼拜七.
