If you pick up a Freetrailer the day before a closing day, you have the Freetrailer for free until the next opening day in the morning when the store opens.
Do you rent for example the Freetrailer on Saturday and the pick-up place is closed on Sundays, you do not pay for the extra day, but you'll just deliver the trailer Monday morning when the store opens. However, if you have an insurance, you must pay for this on the closing day.