Hello, I understand your concern. Usually, Reese s pinworm medicine is given in a single dose. It may be repeated in two weeks, to make sure the parasites have been completely eradicated. It is not very important whether you give it to your child on the 30th or 31st, since there is no risk af drug accumulation from the first dose. It is important however that when a family member is infected with pinworms, in order to avoid reinfection, the entire household needs to be treated too. I hope this helps. Wishing you all the best