Generally, in shows the “largest” time, on shows dates and time, and at usually shows the “smallest” time.
English speakers use in to refer to a general, longer period of time, such as months, years, decades, or centuries. For example, we say “in April,” “in 2017” or “in the 21st century.”
Moving to shorter, more specific periods of time, we use on to talk about particular days, dates, and holidays . You may hear, “I went to work on Monday,” or “Let’s have a picnic on Memorial Day.”
For the most specific times, and for holidays without the word “day,” we use at. That means you will hear, “Meet me at midnight,” or “The flowers are in bloom at Easter time.”
Let’s look at the usage for in, at, on.
Do not use in/at/on before “next” or “last”.
- We will get married next year.
- Their baby was born last March.