Dates and times: “on”, “in”, “at”?

Question

I’m often confused when I speak about times and dates. What is the rule for using on, in, and at in the following sentences?

• I will do it ___ Tuesday.
• We married ___ March.
• He returned ___ the same day.
• Every day ___ the same time, I walk the dog.

Answer 1

Times usually get at.

Everyday at the same time I take a walk.
At 3 PM, I will be having a late lunch.

Days usually get on.

I will do it on Tuesday.
He returned on the same day.

Months usually get in.

We married in March.

Answer 2

From what I know, confirmed by here:

• at for a precise time;
• in for months, centuries, years;
• on for days and dates;

Though I have seen at for days in the plural (e.g. at Mondays), but it looks rather informal in my opinion.