This made the seasons get out of track, since the real first day of spring in western Europe (the equinox - day and night the same length) was happening earlier and earlier before the traditional March 21 as the centuries went by. By the 1500s, it was starting around March 11, ten days 'too early' according to the calendar. So what they did was to move the calendar forward ten days in 1582, and at the same time to make sure it did not happen again. To do this, they made an exception to the previous 'leap year rule' (add February 29 every four years). There would be no February 29 for every year that ends in 00 - unless it could be divided by 400. So the year 2000 was a leap year, because it could be divided by 400, but 1700, 1800, and 1900 would be common years, with no February 29.