September: all of the interns go back to school. These people who exist on the fringes of the system manage to get a lot of work done, possibly because they are free of most of the overhead facing real employees. Once they leave, it's up to the FTEs to own whatever was created, and that doesn't always work. I wish I could have kept some of them and swapped them for some of the full-timers.
March/April: Annual bonus time? That's what it used to be, at least, and I say this as someone who quit in May, and that was no accident. Same thing: people leave, and that dooms whatever they left.