Let's talk about preposition of time.There are three important prepositions of time to be discussed on this forum.
1. (In). Preposition "In" is used for years, months. The clue here, it is always part of year. For example, Teachers of PES celebrated Teachers' Day in October. The clue here is October which is part of the year.
2. On is used for days and dates. In short it is part of the month. The 7 days of the week are parts of the month. For example, The teachers of PES celebrated Teachers' Day on Friday. Friday is part of the month.
And the last one is At. It is used when referring to precised time. Time is part of the day. 9 am, 10 o clock are the examples.