礼拜一 (lǐ bài yī) — Monday

礼拜一 is the everyday spoken word for 'Monday,' used everywhere from casual chats to schedules. It comes from 礼拜 ('week'), so you'll also hear 礼拜二 (Tuesday), 礼拜三, and so on through 礼拜日/礼拜天 (Sunday). It's more colloquial than the formal 周一.