Most time words take 에: 월요일에, 세 시에, 아침에. But 오늘, 내일, 어제, 지금 and 매일 never take it - they stand alone. This small list is worth memorising because the error is very audible.
Examples
- 월요일에 만나요.
- Let's meet on Monday.
- 오늘 시간이 있어요?
- Do you have time today?
- 내일 학교에 가요.
- I'm going to school tomorrow.
The mistake to avoid
오늘에 and 내일에 are wrong - those words take no particle at all.
Where this fits in
숫자, 요일, 시간, Korean A1: Count with Sino-Korean numbers, name the days, tell the time, and talk about parts of the day.