As a host, you have a calendar on your profile showing when you accept visit requests (open), when you don't (closed), and when you already have a visit planned with WWOOFers (planned visit):
You can indicate which months you can typically host WWOOFers on your calendar. Additionally, you may indicate specific unavailable dates within those months:
Finally, "planned visits" are automatically populated when a visit request is confirmed by a WWOOFer. For instance, confirming a visit request from April 1 to April 16 will mark this period in orange. This lets other potential WWOOFers know that you have already accepted a visit request for this period and either there may be other WWOOFers visiting at the same time or you may be full, depending on your availability.