There are many time expressions in English that use in, on and at. It can be a bit confusing to know which one is correct, so let’s look at how we can use them correctly.
In is used with long periods of time like months, years and centuries. For example:
◆ My birthday is in October.
◆ This company was founded in 1953.
◆ We are taking a trip to Hokkaido in the summer.
On is used with days, holidays and dates. For example:
◆ My birthday is on October 3rd.
◆ The package should arrive on Friday.
◆ We went to a party on New Year’s Eve.
Be careful as some time expressions may seem to be the same, but are different. For example, look at the difference between these two sentences:
◆ I like to study in the evening.
◆ I like to study on Monday evenings.
Because “the evening” is a long period of time, we use in. However, “Monday evening” is part of a day, so we use on.