Since we don't have the guaranteed weather of more southern resorts, Monkey Island draws its tourist dollars with fairs, celebrations, and so-called "appreciation days."
And so it is that the Butterfly Migration Festival is held at this time of year, to coincide with the usual week-long or so layover of the butterflies. Most of these butterflies are only on the island for, tops, three or four days, but, since the the monarchs arrive in a staggered fashion over a period of days, the tourists can be here for a fortnight or more, even longer if the beer holds out, all the better for the economy.
The Migration Festival begins around the last Friday in September and runs till the second Monday in October. Hotel space is at a premium, so make your reservations early!