There will be certain dates they avoid. We never play Blose at the start or the end of the season, or around Christmas (presumably the police don't need the extra aggro at that time of year). It also won't be played when it clashes with an FA Cup game as they won't want that game switching to midweek. I'd imagine they'd also be keen that the Sty game, at least, doesn't clash with Pride or St Paddy Parades.
By the time you've eliminated all those, there aren't that many weekends left to choose from, so it's no wonder we always seem to play them roughly around the same time.