First thing in the morning after you've been to the loo and before you've gotten dressed and had anything to eat is the best time. 1lt of water weighs 1kg, so if you weigh at the end of the day you could be adding extra kg's simply because you might have drank several litres of water through the day, plus all the food you've eaten. I only weigh once a month because my cycles affect my weight hugely because of fluid retention, so I have to make sure that it's just after I have my bleed that I weigh.