An interval timeshare is one that's available in a given time period. As mentioned previously, an "interval" in timeshare-speak is just another word for "week". Let me give you an example, and see if this helps clarify the situation.
Let's say that you desperately want to stay in Hong Kong for a couple of weeks in February. There's a festival that you plan to attend, and you don't want to be stuck in a hotel, no matter how nice. You want all the perks of a private residence: private kitchen, bed, and bath, just for starters.
Hypothetical Interval Timeshare
You go through the timeshare classifieds, looking for something available in the time period required. You scour a lot of ads before finding what you need, but there it is: an interval timeshare. Now it's time to negotiate a deal that is workable for everyone.
If you own a timeshare elsewhere, you might try to do an interval timeshare swap: you stay at their place for two weeks, and then they can stay at yours for two weeks of their choosing. If you have nothing to trade, then it's a rental situation. Finally, if this is a place and time you'd like to arrange for every year, you can attempt to buy.
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