Calendars are important in keeping track of the time. The standard calendar that we are using right now is called the Gregorian Calendar which was introduced by Pope Gregory XIII. It is a solar calendar that has 365 days divided into 12 months of irregular number of days. We have used this since 1582.
The Julian Calendar did not properly reflect the actual time it takes the Earth to circle once around the Sun. This calendar produced leap years way too often and holidays were delayed for several days and is out of sync with the events like equinox and solstice.
The papal bull issued by Pope Gregory XIII in 1582, decreed that 10 days be dropped when switching to the Gregorian Calendar. However, the later the switch occurred, the more days had to be omitted. (See table below).
The Gregorian Calendar removed 10 days from the old calendar. This created short months with only 18 days and odd dates like February 30 during the year of the change over.
It took 300 years to make the Gregorian Calendar as the World's Standard Calendar. Turkey was the last country to officially switch to this calendar from the Julian Calendar in 1927.
The Gregorian Calendar uses a more accurate leap year formula which makes it better than the Julian. However, it is still not perfect compared to the tropical year because it is off by one day every 3236 years.
This is just a brief summary on the history of the calendar that we have today. It is important that we have knowledge of the past because we will not have this calendar today if not for the old calendars that we had.