The best way to weigh your self is straight out of bed in the morning. If your weigh your self in the day, during a workout, or at night then this is with water and food which would give you an innacurate reading.
Why is it innacurate? It seems like an accurate reading would be one where you do have food and water in you. Weighing yourself in the morning seems like cheating since you will normally have food and water in you throughout the day.