I. On

Preposition ON is used with:

• days of week (on Monday)
• dates (on 4th of July)
• names of holidays if it contains the word Day (on Christmas Day)
• expressions on the weekend and on weekends (American English)

II. In

Preposition IN is used with:

• year (in 2010)
• months (in January)
• times of the day (in the morning)
• seasons (in summer)

III. At

Preposition AT is used with:

• time (at 2 o'clock)
• certain times of the day (at night, at sunset)
• holiday periods (at Christmas)
• expressions at the weekend and at weekends (British English)