Practise and remember this golden rule:

AT A TIME, ON A DAY, IN A PERIOD

There are some exceptions (see under the Explanations tab), but only a few.

AT a time, ON a day, IN  a period

- I get up at 9 o'clock. At midnight it is dark but at noon it is daylight (midnight= 00:00, noon = 12:00)
- I start school on Monday. Christmas is on December the 25th.
- I go to the sea in summer. I went to Seville in 1992

We also use IN to talk about a future period of time:
- I will go to France in two weeks

EXEPTIONS
- in the morning, in the afternoon, in the evening, at night
- BrE: at the weekend / AmE: on the weekend
- on holiday
- In the past, in the future, at present
- at Christmas (24th Dec - 1 Jan), on Christmas (25th Dec), on Christmas holidays (24th Dec - 1 Jan)
- on + week day + period of day: on Monday morning, on Saturday night. --- on Monday, in the morning.