Cruise lines might offer last minute cruise deals if the staterooms on a cruise aren't full 60 days before the departure date. They cut prices to stimulate demand.
Are last minute deals available for every cruise? No. Far from it. Cruises are popular and the lines don't have to resort to sales to fill ships. Royal Caribbean and Celebrity have stopped the practice altogether.
You can find a last minute cruise bargain in 3 places: at online travel agencies, at a local travel agency, or on the cruise line's own website.
Travel agencies (both online and regular) may buy blocks of unsold cabins at cheap wholesale prices. Then they pass the deals on to their customers. This is why it pays to use a travel agency.
Cruise lines promote last minute deals in their email newsletters, so you should subscribe.