The quick answer

Use at for clock times and a few fixed expressions, in for months, years, seasons and parts of the day, and on for days and dates. Say at seven o'clock, in May, on Monday.

The rule

Choose the preposition by how big or small the time period is. At marks a precise point in time: a clock time (at 7 o'clock), or a fixed expression like at night, at midnight, at Christmas or at the weekend. In marks a longer period: a month (in May), a year (in 2020), a season (in summer), or a part of the day (in the morning). On marks a day or a date: a day of the week (on Monday) or a specific date (on 5th May).

Form

Examples

• I wake up at seven o'clock.
• We have a test on Monday.
• My birthday is in May.
• The shop closes at midnight.
• We go swimming in summer.
• They meet at Christmas.

Common mistakes

• ❌ I wake up in 7 o'clock. → ✅ I wake up at 7 o'clock. Clock times always take at.
• ❌ We have a test at Monday. → ✅ We have a test on Monday. Days of the week take on.
• ❌ My birthday is at May. → ✅ My birthday is in May. Months take in.
• ❌ He wakes up in night. → ✅ He wakes up at night. Night is a fixed expression with at, not in.