When you're trying to lose, maintain or gain weight you need to weigh yourself regularly to check your progress. It doesn't matter what time of day you weigh yourself but it is important to be consistent with the time.
You will weigh more at night than in the morning because in the morning, after you've gone to the bathroom, you will have very little undigested or digested food "waiting to leave your body." If one day you weigh yourself in the morning and the next day you check at night, it'll seem like you gained weight when you really haven't.
The time of day when your weight will be most accurate is in the morning after you've gone to the bathroom. If you have a scale at home, weigh yourself then.