The most common prepositions of time are at, on and in.

Use the preposition in in time expressions, when talking about (i.a.):

• months: The school year starts in August.
• seasons: It is nice to travel abroad in summer.
• general parts of the day: in the afternoon, in the morning, in the evening

Use the preposition on in time expressions, when talking about (i.a.):

• weekdays: What are you doing on Monday?
• dates: I was born on the 22nd of November.

Use the preposition at in time expressions, when talking about (i.a.):

• time or a specific parts of the day: at six o’clock, at noon, at midnight
• holidays: We could go to the beach at Christmas.
• beginning or end of the specific period of time (+ of): at the beginning of July, at the end of the 19th century

Note that prepositions are not used with the following expressions of time: every, last, next, one, this or that:

You’ll be sorry one day.

Other prepositions of time:

• during the lecture
• until the end of March
• since the 1950s
• over the past years
• for a week
• by now