...the Catholic Origin of our Calendar?

The Gregorian calendar is used by almost the entire world and was developed under the direction of Pope Gregory XIII, and introduced in 1582. It was created because the  Julian calendar (established and developed by the Romans) was slightly miscalculated, which meant that Easter, arguable the most significant observance in the Catholic faith, was moving farther away from the Spring Equinox with every passing year. With the introduction of this "new" calendar, Easter was restored to the time of the year as originally celebrated by the early Church.  Learn more about the Gregorian Calendar (including why Orthodox celebrations fall on different days) read more here!
So now you know!