"A Timeshare Interval is the time period that is sold to the buyer. Most timeshares are sold in weekly intervals, though some resorts have longer time periods such as one-tenth year or one-quarter year ownership. Most Timeshares are based on a one-week interval, which is commonly referred to as "Timeshare Week" or "Week". You will see that all advertisements on BuyATimeshare.com will display a week number. This number indicates the week assigned to that ownership, numbered 1 through 52."

http://www.buyatimeshare.com/timeshare-intervals.asp