に

「に」 is used to indicate the specific time at which an action or event occurs. Specifically:
- It is mandatory to use 「に」 when the time is specified by a number (e.g., 8 o'clock, 9:35, December 12th, January 15th, 2023, September...).
- 「に」 is not used when the time is relative, such as using the present moment as a reference point (e.g., tomorrow, the day after tomorrow, yesterday, the day before yesterday, next week, last week, next month, last month, last year...) or for occurrences that are repeated regularly (e.g., daily, weekly...).
- 「に」may or may not be used when the time is relative to a specific point (e.g., one day before graduation, one day after the presentation...) or when referring to a certain duration (e.g., summer, winter, spring break...).
- 「に」may or may not be used with days of the week.