phi

You might want to look at things like your sodium intake and caffeine intake, and see whether those also follow a weekday/weekend pattern, because this sort of up-and-down sounds like more of a water retention and loss thing than a true weight gain and loss. Also, do you drink more or less water and other fluids during the week? Drinking plenty of water can actually decrease water retention, so if you don't drink much, you might try making a conscious effort to get more for a while, and see if that makes a difference.