Century leap year thus the leap day February 29 in those years always falls on a Tuesday (dominical letter BA). The Gregorian calendar yields an average year that currently tracks the annual revolution period of the Earth more closely than the older Julian calendar, in which every fourth year (including end-of-century years) is a leap year. The Julian formula adds too many leap days (3 every 400 years), causing the Julian calendar to drift gradually with respect to the astronomical seasons. Over time, natural events such as the spring equinox began to occur earlier and earlier in the Julian calendar. The Gregorian calendar
Calendar reform months and replaced the intercalary month with an intercalary day to be inserted within February every four years. This produced a noticeably more accurate calendar, but it had an average year length of 365 days and 6 hours (365.25 d). This had the effect of adding about three-quarters of an hour every four years. The effect accumulated from inception in 45 BC until by the 16th century the northward equinox was falling on March 10 or 11. Under Pope Gregory XIII, the leap year rule was altered: century years which are not divisible by 400 would be common years. So