How do you make all your days productive?

I used to think that some days just weren't productive. And I wished I could figure it out at the beginning of the day so that I could spend the day reading or hanging out with friends. Then I realized that each day is different. (Or I'm different each day.) And the key is figuring out what I'll be productive at today.

(I also realized that reading and hanging out with friends is a productive use of time too!)

The problem is I haven't figured out how to tell at the beginning of the day what kind of day it is. So I start out trying to work on what I believe is most important or urgent, and if I don't get anywhere I start switching task types. Maybe it's writing, maybe it's catching up on email, maybe it's crunching those numbers, maybe it's making all those phone calls, maybe it's something mindless like filing expense reports. Usually I find it pretty quickly – or at least by lunch time – but I wish I could some how take my temperature and know what kind of day it is …

How do you figure out what you'll be most productive at on any given day?