Different resorts work differently. Some have fixed check-in days/stay periods, others are more flexible. It also sometimes depends on how far in advance you are booking. For example, most Wyndham resorts require you to book a 3, 4, or 7-night stay that either begins or ends on a valid check-in day (usually Friday, Saturday, or Sunday). But, within 90 days of checkin, you can book anything that is still available, starting and ending on any day, as long as it is two nights or more.