I imagine most people know how they can have large weight fluctuations, even within a 24 hour period. I read up on this topic yesterday and today. I found a couple interesting graphs that show results from a test on this. For those who love graphs as much as I do, you can see them at the following links:

File:PSM V58 D220 Human body weight variations over a 24 hour period 1.png - Wikimedia Commons (Days 1 & 2 for a particular test subject)

File:PSM V58 D221 Human body weight variations over a 24 hour period 2.png - Wikimedia Commons (Days 3 & 4 for same test subject)

As for the second graph, for anyone not immediately familiar with metric weight measurements, I added the following blurb when I posted it in my blog:

"On Day 3 at breakfast, test subject weighs 67.9 kg (149.7 lbs). After supper (same day), they weighed 69.55 kg (153.3 lbs). That shows as much as a 1.65 kg (3.64 lb) gain in same day. Weight decreases notably by breakfast the next day (Day 4)."

The above certainly provides an argument against weighing oneself every day, or especially multiple times per day. In fact, from my experience, monthly weigh-ins usually provide the most reliable evidence of true fat loss.