anordinaryguy,
You are right - with prepositions of time, we need to use 'on' before days and dates.
Also remember that 'at' is used to denote a specific point in time. For example, it is appropriate to say, "I plan to start at noon". However, it is not appropriate to say,"I plan to start at the evening". Rather, the sentence should be - "I plan to start in the evening". The reason is that noon is a specific point in time but morning and evening are both periods of time characterized by a common term. Similarly, 'in' is used before long periods. Example "The Berlin wall collapsed in 1989".
So, to sum up, the rules are:
1. Before a date that is mentioned (example: 4th of July, Friday, Monday) use 'on'.
2. Before a day that is not mentioned (example: yesterday or today) use no preposition.
3. Before a period in time, use 'in'.
4. Before a specific point in time, use 'at'