in vs on

"On" a date or a day. On May 23rd. On Monday. On the third day of June. On my birthday.

"In" a period. In May. In the first week of June. In 2015. In the summer. In the 1990s. In the 20th century. In the morning/afternoon/evening/night.

"At" at a particular time. At 12:15 PM. At noon. At midnight. At the time agreed. At the time of death.

Also "on" multiple days. The shop is open on Mondays. The bus runs on Mondays, Wednesdays and Saturdays.