The Misteri d'Elx (Mystery Play of Elche) is a medieval liturgical drama telling the story of the Assumption of the Virgin, performed every year on 14 and 15 August in the Basilica de Santa Maria. It combines music, theatrical machinery, and elaborate staging — including a gilded araceli (a mechanical cloud device) that descends from the dome to carry the Virgin toward heaven — in a form that has remained essentially unchanged since the 15th century. In 2001, UNESCO inscribed it on its list of Intangible Cultural Heritage of Humanity.
The performances are free to attend but the basilica fills quickly; if you're planning to be in Elche in August, arriving early matters. There are also abbreviated rehearsal performances on earlier dates in August (the assaig) that are worth attending if you can. The rest of the year, the Museu de la Festa at Carrer Major de la Vila 27 has a permanent exhibition about the play's history, costumes, and machinery — a surprisingly interesting visit for something that exists only to document a performance that happens once a year.