The months that have 31 days are January, March, May, July, August, October, and December

. One way to remember this is the rhyme "Thirty days hath September, April, June, and November; All the rest have thirty-one, Excepting February alone, And that has twenty-eight days clear, and twenty-nine in each leap year"

. Another method is to use your knuckles: if the month is on a knuckle, it has 31 days; otherwise, it has 30 or less days