Vasilissa ergo gaude ("Therefore rejoice, princess") is an isorhythmic motet by the Renaissance composer Guillaume Dufay. In terms of its subject matter, it is sometimes grouped together with Lamentatio sanctae matris ecclesiae Constantinopolitanae, and which are generically called Dufay's Byzantine motets. Its composition was occasioned by the marriage on 21 January 1421 of Cleofa Malatesta, daughter of Malatesta di Pandolfo, to Theodore II Palaiologos, son of the Byzantine emperor Manuel II and Despot of the Morea. It has been surmised that the actual motet was composed in 1420 and is perhaps the earliest example of this choral form if not the earliest work attributed to Dufay. In earlier scholarship it was assumed that the motet's first performance had taken place on May 19, 1419, dur