I've always prefered a daily weigh-in using the same variables (pre-breakfast, post-morning wee). You'll get smaller fluctuations and then get a more accurate average trend line. For instance I've found out that for some reason I always lose weight consistently through Tue-Sat and then it increases on Sun/Mon. I'm also trying to lose weight at the moment and am struggling to get the balance right between eating enough to fuel recovery and maintaining a calorie deficit. Generally I do OK on days when I'm training, it's the recovery days that I want to stuff my face!
As above will probably be water retention and if you're doing bigger miles on a Sat/Sunday you'll then retain a bit more as no doubt you're eating a bit more and when not working you usuaully find your body will naturally try to recover as much as possible when the stress is less. Even if you go over on non training days it's not a problem as long as your week is in deficit. Usually the body will want to carry on ticking over even on the rest days so good to use these to get some proper recovery in, as you can't cut constantly.