as soon as i learned the knuckle trick, it was all i ever needed to quickly remember how many days are in each month! as demonstrated in the photo below, start with two fists. the leftmost knuckle represents january, then the valley is february, the next knuckle is march, etc... all the months on knuckles have 31 days. the valley months have 30 (or how ever many february has in a particular year!). very useful!
image found at top of the lawn.