"A leap year is never a good sheep year!"
also might be considered carefully, when solving highly sophisticated problems, what Pope Gregor really did when he introduced his new calendar based on "In omnibus solemnitatibus christianis non ignoramus paschale sacramentum esse principium, (sermo 47)":
In the Gregorian calendar, the current standard calendar in most of the world, most years whose division by 4 equals an integer are leap years. In one leap year, the month of February has 29 days instead of 28. Adding an extra day to the calendar every four years compensates for the fact that a solar year is almost 6 hours longer than 365 days.
However, some exceptions to this rule are required since the duration of a solar year is slightly less than 365.25 days. Years which are evenly divisible by 100 are not leap years, unless they are also evenly divisible by 400, in which case they are leap years. For example, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. Similarly, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 will not be leap years, but 2400 and 2800 will be. By this rule, the average number of days per year will be 365 + 1/4 − 1/100 + 1/400 = 365.2425, which is 365 days, 5 hours, 49 minutes, and 12 seconds.
The Gregorian calendar was designed to keep the vernal equinox on or close to March 21, so that the date of Easter (celebrated on the Sunday after the 14th day of the Moon that falls on or after 21 March) remains correct with respect to the vernal equinox. The vernal equinox year is about 365.242374 days long (and increasing), whereas the average year length of the Gregorian calendar is 365.2425.
The marginal difference of 0.000125 days means that in around 8,000 years, the calendar will be about one day behind where it is now. But in 8,000 years, the length of the vernal equinox year will have changed by an amount which cannot be accurately predicted (see below). Therefore, the current Gregorian calendar suffices for practical purposes, and Herschel's correction (making 4000 AD not a leap year) will probably not be necessary.
(taken from wikipedia)