I weigh unofficially most days, but tend to decide an "official" day or two each week. It's not always the same day, it depends what I have planned,and I'm not sure it makes too much difference really.

My official day is Sunday, first thing in the morning, although I might weigh mid-week if I'm concerned. Sunday is my "feast" day (not to be confused with a binge day). Then, I have the rest of the week to even out Sunday's little indulgences before weighing again.