The international calender that we fall under is called the Gregorian calender and was devised by a Clabria doctor named Aloysius Lilius. Six years after his death his brother presented the manuscript to Pope Gregory XIII, it was then given to a reform commision and modifide by Christipher Clavius a German Jesuit Mathematician before being implemented on October 4, 1582. The Jesuit order or the "Society of Jesus" (IHS) is the largest society of male religious order in the Catholic Church, and still resides today with 18,815 members—13,305 priests, 2,295 scholastic students, 1,758 brothers and 827 novices—as of January 2008.
The founder of the IHS was a Spanish knight by the name of Lgnatius of Loyola after reading Meditationes de Vita Christi and claimed to have had a vision of the Virgin Mary and the baby Jesus at the shrine of "Our Lady of Montserrat" while living as a hermit in a cave. He eventully claimed him elf as the first Superior General to the order. the Meditationes de Vita Christi original author's are unclear but was a book written about the life of Christ depicted from the moments of the Gospels influenced art.