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 .