The word extravaganza comes from the Italian word estravaganza (or stravaganza), which was an elaborate literary or musical production. When the word appeared in English in the mid-18th century, English speakers turned the estra- into extra- because after all, an elaborate entertainment could certainly be called something "extra." Aida at the Baths of Caracalla, complete with horses, astonishing costumes, and faux-Egyptian temples — now that's an extravaganza.