Even though I have a set "weigh-in" day, I always weigh myself the day before as well to make sure I'm on the right track. So if I see, say 165 on Thursday, then 160 on Friday.... I'd probably average the two, and call it good. However, if I see 165 on Thursday and 164.8 on Friday-- I know I'm 164.8
I had that same sort of troubles a while ago-- I went from 173 to 159 in one night-- my scale had a calibration problem (it was convinced I was 159 ALL DAY-- no matter what I wore or what I ate..) So I checked my weight at a girlfriends house to make sure it was the scale that was crazy, not me
Hope this helps!! ^_^