I suppose it's a necessary evil, but signing up for things is so often a huge headache; somebody always feels shafted. It is considered the epitome of fairness and free choice, especially in the education world, but no matter the rules of play, people are dissatisfied with the days they have to do hall duty, the hours they get in the computer lab, the month they have to provide snacks, whatever.
First come first serve? Whoever got there first had inside information and whoever was last had an unavoidable conflict. Limit the days? So and so got his friend to sign up for him and that one over there booked the time and never showed! November? That's Thanksgiving! January? It'll probably snow.
What can you do?
Nine or ten years ago, my sister-in-law gave me the European Board Game of the Year (I know, right?) for my birthday. The complexity and critical thinking that Settlers of Catan requires made it perfect to play with my nephews. Before the game starts, players roll a die to see what order to place their two villages on the board. Here, both strategy and luck are involved, because whoever goes first only gets to put one village down, and they will go last in the next round.
Every school year at this time, the teachers on my team have to sign up for eleven hours of after school Homework Club, and taking a page from those hearty settlers of Catan, we do just as they must: half up front and the other half in reverse order. Getting the days you want requires strategy and luck, but nobody complains.