Voici un petit résumé :

In :

Years (in 1999)

Months (in January)

Seasons (in the summer)

Parts of the day (in the morning/afternoon/evening)

On :

Days of the week (on Wednesday, on Friday morning)

Special days (on my birthday, on Christmas Day)

At :

Times of the day (at 4.00, at midday, at midnight)

'the weekend' (at the weekend)

Holiday periods (at Christmas/Easter)

Meals (at breakfast/lunch/dinner)

Parts of the day 'night' (at night)