If it's offered again, there is a certain time in which you must book it (usually a span of a month or two), and a certain time of the year when you have to go (traditionally late August through Sept). You have to buy at least 1 park pass per person and there is a minimum and maximum night stay at a on-property hotel. I think it was 3-10 days. No room discounts, either (like codes or CM rates).
BUT, they might change the restrictions, dates, offer, or even not do it at all.
If I remember right, the past offers came out around March... But I could be wrong on that.