The calender modifications I've discussed so far only involve the civil calendar. Although today we think of the Gregorian reform as a correction of the solar (Julian) calendar, at least as important to those who enacted the reform was the correction to the lunar calendar used to track the Easter months. In attempts at calendar reform before the Gregorian, the lunar months were most frequently the target, probably because it was felt much easier to reform the part of the calendar only used by churchmen, rather than one deeply entrenched in civil life. A little known canon of the Council of Trent (1542) actually did reform the lunar cycle by adding 4 days and decreeing an extra lunar leap day every 300 years. The new breviaries with these cycles weren't published until 1568, and were superseded by the Gregorian reform 12 years later. As mentioned above, the 19-year cycle that eventually became standard had a small error, which the Gregorian reformers measured to be 1 day in 312.7 years. In the Gregorian calendar, the approximation of 8 days in 2500 years is used. One lunar leap day is added every 300 years (400 the last time in the cycle).
Many Protestant countries (but not England), eventually reformed their civil calendars, but switched to a different method of calculating Easter. The so-called "improved" calendar used astronomical tables to find the Easter full moon, and, while it generally matched the Gregorian tables, differed in a few years. Most countries had given up the improved calendar for the Gregorian easter by the end of the eighteenth century.
Orthodox churches also refused to accept the Gregorian method for Easter. They, however, continue to calculate Easter in the Julian calendar. Some orthodox churches never adopted the Gregorian calendar at all, and so also use the Julian calendar for fixed holidays like Christmas.