What is the proper usage of 'at', 'on', and 'in' for prepositions of time?

The prepositions of time are used as follows: 'at' for specific hours and holidays without 'day' (e.g., at 6:00, at Christmas), 'on' for specific days and dates (e.g., on Monday, on 5th May), and 'in' for months, years, centuries, and long periods (e.g., in March, in 1990).