Well, it works both ways. I had a family give notice the night before my vacation and since I didn't have any computer access I didn't know for 11 days about that notice until I was about to go back to work. Big surprise. It happens to us all the time but we want to keep a good reputation in the business so we worry about being super fair to our clients.

Logically, I think you can give the 20 business days notice as per your contract and you are fulfilling your obligation according to your contract. You are open and it's business as usual for you. It isn't your fault that a family's vacation falls during that time.