In the new system, there were twelve monthly divides of similar, although unequal lengths. February, the second month, was the only month to contain 28 days, so the added day in leap years was solidified as 29 February. Leap years exist once every four years, and only occur in years that are divisible by four. There is an exception to the divisible-by-four-rule, and that is if the year ends in -00. Even though years ending in -00 are divisible by four, it is not a leap year. However, there is one more exception to the exception. Years ending in -00 that can be divided by 400 are in fact leap years.
The Gregorian calendar was decreed valid by Pope Gregory XIII on 24 February 1582, but was not accepted by any European nations until October of that year. The first countries to begin mandating the use of the calendar were Spain, Portugal, Italy, and the Polish-Lithuanian Commonwealth. France and Holland quickly followed, both adopting the new method before the end of 1582.
Non-Catholic countries understandably had reservations about accepting a calendar specifically aimed at furthering the goals of the Roman church. Europe was heavily involved in the Protestant reformation when the calendar was proposed, and anti-Catholic sentiment long postponed the unification of Europe under a common calendar. Eventually, the benefits of a common date system became impossible to dismiss, and while the calendar had been created at the whim of the Catholic Pope, its scientific basis made a considerable amount of sense.
It took several centuries, but by 1929 most countries in the world had begun using the Gregorian calendar. China, the last nation to adopt the system, technically accepted it beginning in 1912, but civil unrest left the calendar question undetermined until the unification of the country in 1929. Other countries, such as Japan, accepted the use of the calendar for dealings with the western world, but still maintained local systems in place for centuries.