The idea behind alternate side parking is that everybody parks on the even side of the street (the side on which the buildings with even addresses are) on even numbered days and on the odd side on odd numbered days. This is not as simple as it sounds, since the day in question is the following day (it's overnight parking that is being regulated) and you get consecutive odd numbered days at the end of months with an odd number of days. As I understand it, it's a snow-plowing thing, assuring that at least one side of the street will be fully plowed if overnight plowing is a necessity.