it is the day to get you stuff done.
person a: "what day is it?"
person b: "It's the day to do the day, i gotta go to work, pick up my girl, and clean, and mow, and school."
When something is such a good deal or idea that you would gladly take it at any time
Them: $500 a month utilities and internet are included
You: I'd do that all day!