I think it matters exactly how the payment you make is calculated.
Do you pay per day or per hour, so that each month’s bill flexes up and down depending on Bank Holidays, exactly where weekends fall, exact number of days per month etc? Or if per hour, those children booked in until 5 pay less per month than those attending until 6?
(My children’s after school club billed like this - paid only for days and hours booked, variable per month)
If that is the case, then asking you to pay a full day / full number of hours for a day when the nursery is closed is unreasonable, and you should subtract that amount from what you are billed.
On the other hand, if you pay 12 identical monthly instalments, or 3 termly or 6 half termly, then that seems to me to be reasonable - you are paying ‘the annual cost of the use of nursery when it is open’, divided into equal instalments for convenience, and that annual charge will be for ‘when nursery is normally open’, taking into account BH and other occasional closures or reduced hours.