The Gregorian Calendar
The Gregorian calendar was an amendment of the Julian calendar as proposed by Pope Gregory XIII in the 16th Century. The Julian calendar accounted for the difference between the calendar year of 365 days and the tropical year of approximately 365.24 days by inserting an extra day into the calendar every four years. This caused problems, however, after several hundred years as the application of this rule every 4 years still caused the calendar to become out of synchronisation with the solar year. By the 16th century it was discovered that the calendar was some 10 days out of synchronisation, e.g. Christmas Day had drifted to the time of year that was originally January 4th. If this continued then, eventually, Christmas Day would be celebrated in the height of the northern hemisphere's summer!
It was calculated that this discrepancy could be eliminated by first losing the 10 days that had accrued, to get the calendar back in synchronisation again. Furthermore, it was realised that on average there were three leap years too many roughly every 396 years, so it was decreed that century years (e.g. 1700, 1800) would NOT be leap years unless they are divisible by 400 (e.g. 1600, 2000).
The catholic countries of Europe changed to the new Gregorian Calendar in 1582 whereas other countries did not change until 1700 or later. Great Britain and her colonies did not change until 1752, by which time 11 days had to be chopped from the calendar for that year. The Julian calendar, if still used, would now be 13 days behind the Gregorian, rising to 14 days in the year 2100 following the next skipped leap year in the Gregorian system.
Although the Gregorian calendar is more accurate than the Julian, it is still not as accurate as it could be. There is still a small error between the average length of the year and the length of the actual year, which will amount to a whole day in just over 3,000 years. There may need to be a leap day dropped in around the year 4,000 but this has not been arranged and may turn out not to be necessary due to changes in the Earth's orbit. Also, during the 200-year period between the dropped leap years (i.e. the period when there is a century year that has a leap year, such as the year 2000) the calendar slips as compared to actual solar events by as much as 2.15 days. For example, the winter solstice occurs on 23rd December in 1903, i.e. just after the dropped leap year in 1900, but arrives as early as 20th December in 2096, i.e. juest before the next dropped leap year in 2100. The image below illustrates just how pronounced this slippage is, characterised by the erratic nature of the blocks of dates. Other calendars, such as the Iranian calendar, which has eight leap years every 33 years, is more accurate and such slippage is not so evident, as demonstrated in the second image below.