"The song got its name from Ederlezi, which is a festival celebrating the return of springtime, especially by the Romani people of the Balkans, and elsewhere around the world. Ederlezi is the Romani name for the Feast of Saint George. It is celebrated on 6 May [O.S. 23 April] (occurring approximately 40 days after the spring equinox). The various Balkan spellings (Herdeljez, Erdelezi) are variants of the Turkish Hidirellez."
It's a beautiful song, eminently suitable for Saint George's Day. There are many fine recordings online.