Île des Faisans is part of France from February to July only. Isla de los Faisanes is part of Spain from August to January only. They’re the same island.
The northern border between Spain and France runs along the Bidasoa River into the Bay of Biscay. Close to the coastline is a tiny river island known (in English) as Pheasant Island. No-one lives there and no-one is allowed to visit there, but since 1659 it has been under the control of both France and Spain. By mutual agreement, each country controls the island for half of the year: France from the start of February until the end of July, and Spain from midnight of August 1st until the end of January.
The agreement that produced this unorthodox sovereignty arrangement was the Treaty of the Pyrenees, which ended a war between Spain and France that had run for 24 years (and is today considered part of the Thirty Years War). In fact, that treaty was negotiated and signed on Pheasant Island – and a year later it was also where one of the signatories met his future wife. It was an arranged marriage, part of the treaty in fact, King Louis XIV of France meeting Maria Theresa and her father, King Philip IV of Spain. Not too bad for an island that’s two hundred metres long and only forty metres wide at its widest point.
So who gets the better end of this deal? Well, February to July is 181 days (182 on a leap year); August to January is 184 days. So Louis XIV lost three days but gained a wife.
[Thanks to Gareth E. for suggesting this topic.]