Yep weight can fluctuate up to 12 pounds a day!
Wow, so all those who want to lose weight, weight your self after you go potty

As Pointed out by Sherasi, Bowel movements have a large part to do with the fluctuation, so to does urination. Followed by sweating.
Ideally if you are going to weight yourself everyday (Why you want to waste your time doing so since significant weight loss/gain takes place over a period of several days) Do so at the same time, after the same routine (Say after the potty run in the morning before you have breakfast/coffee (Remember, coffee is the most important meal of the day

)
Ideally, if you are on weight watching diet, you would want to only weight yourself once a week or bi monthly.
However, weight doesn't mean anything. I know a guy who stands 6'2 weight in at 300 lbs. But no one would call him fat, see he is a body builder and has a lot of lean muscle mass.
Getting a Body Fat Percentage measurement is your truly best method of knowing if you are over weight or not.
This is a complicated measurement and can not be done at home. Either your Doctor can do this, or any gym with a good trainer.
BMI calculators are worthless, again, I use the bodybuilder as a reference. Take the two measurements I gave you (weight and height) and according to the BMI Calculator the dude is obese.
In the end weight is meaningless unless you are 50 or more pounds over weight, then it is just a general gage.
Mind, most people end up losing lean muscle mass on any diet - thus we can not honestly say that a loss of ten pounds means one has lost ten pounds of fat.
Cheers
David