In, on, at, for
I usually have my holiday in July.
She comes at 9 every day.
I play tennis on Monday.
I work for 10 hours every Tuesday.
When we speak about some longer period of time, we use in.
When we speak about one moment in time, we use at.
When we speak about a day, we use on.
When we do something for some time, we use for.
You can see more examples in the book Robinson Crusoe.