One point in time

On is used with days:

• I will see you on Monday.
• The week begins on Sunday.

At is used with noon, night, midnight, and with the time of day:

• My plane leaves at noon.
• The movie starts at 6 p.m.

In is used with other parts of the day, with months, with years, with seasons:

• He likes to read in the afternoon.
• The days are long in August.
• The book was published in 1999.
• The flowers will bloom in spring