How do I tell which months have 30 days and which have 31 without reference to a calendar?
type: trick
reliability: 
- very high
understandability: - very high
time saving: 
- small gain
usefulness: - very high
difficulty: - easy
required skill: 
- newbie
overall: 41 of 48 points,
Either 30 or 31, it can be frustrating, especially in the long run. There is a trick to tell accurately which has which without memorization. The trick is as follows:
- With both of your hands, make a fist and curl your fingers in toward your palms so they resemble the image below.
- Put both hands together without criss-crossing them.
- High spots represent more days as more is a higher quantity, thus a higher point.
- Low spots represent less days, thus a lower point.
- Ignore the gap between the two hands as you know July and August are the only two months consecutively within the same year that have 31 days.
- February is the odd ball as you should know. It has either 28 or 29 days, depending on the year number.
- If the year number was not a multiple of four [i.e. 2003], February has 28 days.
- If the year number is a multiple of four [i.e. 2004], February has 29 days.
- If the year number is a multiple of 100 but not 400 [i.e. 1900, not 2000], February has 28 days.
- If the year number is a multiple of 400 [i.e. 2000], February has 29 days.
What's with the strange pattern with the 4 and 400? The true number of days in the year is 365.242199 days*, not 365 1/4 [or 365.25 as a decimal to help you compare] as you may know of it better. Every year, about 1/4 of a day is "lost". Every 4 years is when a leap year occurs thus replacing those four lost days, however, upon doing so, and repeating it for 100 years, you'd have 0.7801 lost days. 4 times that, for 400, is 3.1204 lost days. It'll be around year 3360 before another special adjustment needs to take place.
Footnotes:
* If you do a search in google for "days in year", you'll see this 365.242199 number. If you're wondering how many times a clock goes "tick tock" every year if every "tick" was one second, it'll make 31,556,925.9936 ticks, or, which is super close to 31,556,926, or, as I commonly say, 31 5/9 million.
