@Rebecca1226 It depends on how far ahead an individual host has their calendar open for. Some have it open a year in advance, some only 3 months. But you can always send an Inquiry message (click on the "Contact host" in the listing page) to a host to see if they will take a reservation for the dates you want.
You do have to enter available dates when you do this, but you can just enter any random dates that you see available on their calendar, then explain in the message body regarding the actual dates you want.