pageant (n.) Look up pageant at
late 14c., "play in a cycle of mystery plays," from Medieval Latin pagina, of uncertain origin, perhaps from Latin pagina "page of a book" (see page (n.1)) on notion of "manuscript" of a play.

But an early sense in Middle English also was "stage or scene of a play" (late 14c.) and Klein says a sense of Latin pagina was "movable scaffold" (probably from the etymological sense of "stake"). With excrescent -t as in ancient (adj.). Generalized sense of "showy parade, spectacle" is first attested 1805, though this notion is found in pageantry (1650s).