From wikipedia: 40 (forty) is the natural number following 39 and preceding 41. In English, forty is the only number whose constituent letters appear in alphabetical order.
Forty is an octagonal number, and as the sum of the first four pentagonal numbers, it is a pentagonal pyramidal number. Adding up some subsets of its divisors (e.g., 1, 4, 5, 10 and 20) gives 40, hence 40 is a semiperfect number.
Today also happens to be my fortieth birthday…