The most common prepositions of time are in, on and at.
In is often used when we talk about a time inside a large period - e.g. 'My birthday is in September.'
On is often used when talking about specific dates - e.g. 'My birthday is on September seventeenth.'
At is used when talking about time - e.g. 'I was born at 18:00 on September seventeenth.'
Months (September), seasons (Autumn), years (1986) and phrases like 'the morning' are usually used with in.
Festival days (Christmas Day, Easter Sunday), days of the week (Saturday) and dates (25th December) are usually used with on.
Time (18:00 or 6 o' clock), festival periods (Christmas, Easter) and the phrase 'night' are used with at.