If they paid for them then yes for determining the day rate. What you might want to do is include in your contract something to the effect of what the day rate will be and have it apply to any day any month and that it will be deducted from the following month.

Some caregivers also have 3 floating paid holidays in their contract to cover things such as this or a sick day. That depends on the norms for your area though on what parents will accept.