A Christian calendar can be any calendar used by various Christian Churches for the Church year, called "liturgical year". For example, it can be used to find out what day Easter will be on, even though it is not the same in every Church.
The church year is divided into seasons like Lent, Easter, Pentecost, Advent and Christmas, with other weeks being seen as ordinary time.
The Gregorian calendar, named after Pope Gregory XIII, is now a civil calendar that is used in many countries around the world.
Catholic, Anglican and Orthodox Christians also celebrate Saint's days - here is a Catholic list
First page of the papal bull Inter gravissimas
Detail of the pope's tomb by Camillo Rusconi (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar.
Christopher Clavius (1538–1612), one of the main authors of the reform.