I've normally found that working girls also have friends, family, plans and dreams. It's a bit hard to turn down that invite to your grandmothers garden party (or whatever) without raising some suspicions. Everyone gets invites on a bank holiday.
So, I think it'll be a bit harder to find a girl over the weekend.
That, plus more guys finding that extra couple of hours free for a punt.
My advice? If you're in need, find a masseuse who'll give a little extra if you're stuck over the weekend.