Gregory, whose real first name was Ugo*, followed Pius V's lead and worked to reform the church. He's one of the few popes that has had a world-changing impact: he gave us the Gregorian calendar.

Before Gregory, Europe used the Julian calendar - which Julius Caesar put into effect in 45BC. That calendar lost a bit of time, but Gregory's contemporary Johannes Kepler pinpointed the errors and advised the church on a more accurate system. While Gregory promoted the new system, it took a few centuries to catch on throughout Europe.

*yes, Ugo. Awesome, no?