Original: It was first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, after whom the calendar was named, on 24 February 1582 by the papal bull Inter gravissimas.
Simplified: It was first suggested by the Neapolitan doctor Aloysius Lilius, and was made official by Pope Gregory XIII, for whom it was named, on February 24, 1582.