The Julian calendar based on the Roman calendar that had been in use for over 1500 years was not correct. In the Julian calendar a year was 365.25 days (365 days 6 hours) but should have been 365.2425 days (365 days 5 hours 49 minutes 12 seconds). The spring equinox occurs on/about March 21 but under the Julian calendar it had gradually moved back to on/about March 11. Since the date for Easter was dependent upon the equinox, it, too, was out of sync. Gregory dropped those ten days and moved the date back to March 21 and readjusted the calendar's leap years based on the new calculations. The reform was made in 1582 by Pope Gregory XIII.
Pope Gregory XIII
It is named after Pope Gregory XIII, though he did not invent it.
Pope Gregory XIII revised the Julian calendar in 1582.
Pope Gregory XIII in 1582.
A calendar introduced by Julius Caesar in 45 BCE, which was later corrected by Pope Gregory XIII in the Gregorian Calendar.
It is called the Gregorian calendar after Pope Gregory XIII and was introduced in 1582
The Gregorian calendar was initially decreed by Pope Gregory XIII on 24 February 1582.
The Gregorian calendar, named after Pope Gregory XIII, who rectified errors in the Julian calendar, which was the previously accepted calendar.
Pope Gregory XIII
The Gregorian calendar was introduce by Pope Gregory XIII in 1582 to correct the discrepancies that had built up with the Julian calendar. Eleven days were removed.
The Gregorian calendar was an adaptation of a calendar proposed by Aloysius Lilius in 1582. However, the calendar is named after Pope Gregory XIII who introduced this calendar by a papal bull. It was a reform to the Julian calendar.
Properly speaking, there is no "English" calendar system. You probably mean the predominant Western Calendar (also called the Christian Calendar). This calendar system is best called the "Gregorian Calendar", after Pope Gregory XIII who introduced the calendar system in 1582.