Why do we call the calendar we use today the Gregorian Calendar?

The Gregorian calendar was the idea of Pope Gregory XIII.  The calendar was named after Pope Gregory XIII.  The calendar's initial introduction occurred in the 16th century, in the year 1582.  Prior to the adoption of the Gregorian calendar, European nations used the Julian calendar.  This calendar was introduced under Julius Caesar.  This calendar had certain errors.  For example, a miscalculation caused the calendar to not be consistently aligned with the seasons.

Pope Gregory XIII thought that it was important for Easter to be at around the same time every year.  The Council of Nicea had occurred in the springtime, and it was recorded that Easter had taken place close to this event.  

The Gregorian calendar was first adopted in Catholic nations.  Predominantly Protestant nations were slower to adopt the Gregorian calendar, though they did so eventually.  When Great Britain adopted the Gregorian calendar in 1752, eleven calendar days were lost.