What's a good memory trick for days?

The best trick is the Knuckle Method make a fist and count across your knuckles:

• Raised knuckle = 31 days
• Valley between knuckles = 30 days (or 28/29 for February)

Start from the first knuckle (January), go across to the last (July), then start over from the first knuckle again for August through December.

Rhyme alternative:

"30 days have September, April, June, and November. All the rest have 31, except February alone, which has 28 days clear and 29 in each leap year."