@Vanessa385, your property belongs to you, not to the guest.
If you had changed your availability after the guest made a reservation, that would be one thing.
But the guest made a reservation knowing the earliest day/time they could arrive already. They can *ask* if you can accomodate something different, but they have no right to make demands.
Continue to stand firm!
How would they feel if *you* decided to change things and and said "hey, could you check out a day or two earlier than we agreed?"