It's whatever you want it to be.

I would try to pick a time that has the best chance of being consistent from weigh-in to weigh-in. It's best to do the weighing first thing in the morning right after visiting the bathroom. That way you have had several hours of continuous rest and fasting. I picked Monday mornings for my "official" weigh-in because I have more control over my eating patterns on Saturday and Sunday (usually).