You use ''in'' if you speak about a year when something happened, e.g. I met her in 1996.
If you say a particular date, you use ''on'', e.g. I met her on the 12th of September in 1996.
You also use ''in'' with morning, afternoon and evening, e.g. in the morning, in the afternoon.
BUT! You say 'at night'.
Also you use ''at'' when you speak about time: at 5 o'clock in the morning.
Altogether it might look like this: I met her at 5 o'clock in the morning in 1996.