“The best number of all, really, is 40. It means eternity. Or, sometimes, everlasting time, or all the time that there is. Many eastern churches have forty windows in the circumference of their domes, to show that the domes represent the everlasting heavens. The Jews reckoned that if you fast forty days, you’ve fasted long enough to atone for all your sins; forty days of rain is enough to wipe out everything on Earth. This is distinct from 3, and from 6, which also means enough time, but in different ways; 40 bears the connotation of forever, of permanence, of completeness. If Christ had been in the tomb for forty days, that would have meant that he’d be gone until all possible time was gone. But staying dead three days is staying dead enough days; by staying with us in his human body for forty days after the Resurrection, he showed us that he’d be with us forever.”
From Why Do Catholics Do That? by Kevin Johnson.
Great chapter on numbers.