A preposition of time is a preposition that allows you to discuss a specific time period such as a date on the calendar, one of the days of the week, or the actual time something takes place. Prepositions of time are used to discuss times.
At – This preposition of time is used to discuss clock times, holidays and festivals, and other very specific time frames including exceptions, such as “at night.”
In – This preposition of time is used to discuss months, seasons, years, general times of day, and longer periods of time such as “in the morning.”
On – This preposition of time is used to discuss certain days of the week or portions of days of the week, specific dates, and special days such as “on Sunday.”