The host is the one responsible for cancelling your reservation since the host is the one who did not keep their calendar up to date. I would instruct the host they need to cancel your reservation ASAP so your money is refunded. If the host does NOT cancel your reservation, you need to call Airbnb to ask them to cancel on the host's behalf. Note: You should get a FULL refund, as this was NOT your fault. Do no cancel the reservation on your own, as you will NOT get your Airbnb fees back.
You'll need to Google for the Airbnb contact number in your country.