@Anne1740 It's really your responsibility as a host to make sure your availability is set the way you want it to be. Having a one day minimum set for a long holiday weekend and then wanting to decline a reservation and hold out for a longer one isn't very professional and declines count against you.
You coulod explain to the guest that you are a new host and didn't realize how to set your availability as far as minimum days and see if they would be amenable to cancelling the Request so you don't have to decline.
And are you sure this was a Booking Request, rather than an Inquiry? Because you don't have to either pre-approve or decline an Inquiry- just messaging back within 24 hours is all that's required. You'll get prodding messages from Airbnb to pre-approve, but you can safely ignore those as ong as you've messaged the guest back.
If it's a Booking Request, then yes, you do have either Accept of Decline within 24 hours.