When combined, 礼拜 (lǐbài) means “to worship” or a “(religious) service.” Adding 天 (tiān) turns it into “the day for worship,” a direct reference to the Christian Sabbath, or Sunday.
The existence of 礼拜天 (lǐbàitiān) is a direct linguistic footprint of Western cultural exchange, specifically from Christian missionaries. While most days of the week in modern Mandarin follow a simple numbering system (e.g., 星期一, xīngqīyī, for Monday), the terms for “week” and “Sunday” have more varied and culturally rich origins. In Western cultures, the name “Sunday” is pagan in origin (“Sun's day”). However, in many Romance languages, the word for Sunday derives from the Latin Dominus, meaning “the Lord's Day” (e.g., Spanish domingo, French dimanche). 礼拜天 (lǐbàitiān) is culturally much closer to this “Lord's Day” concept, as it explicitly refers to religious observance. Despite this origin, its usage in modern, largely secular China is completely mainstream. Using 礼拜天 doesn't imply you are religious, any more than saying “Thursday” in English implies you worship Thor. It has become a standard, if slightly informal, part of the daily lexicon, valued for its warm, relaxed feel.
礼拜天 is primarily used in spoken, informal contexts. It feels a bit more personal and traditional than its common synonym, 星期天 (xīngqītiān).
While the full “礼拜” series (礼拜一 for Monday, 礼拜二 for Tuesday, etc.) exists and is common in some southern dialects, it is not standard in Mandarin. For a beginner, it's safest to learn the 星期 (xīngqī) system for all other days and use 礼拜天 as a common alternative for Sunday only.