Complex sentence: Most leap week calendars take advantage of the fact that 400 Gregorian Calendar years have exactly 20,871 weeks, so with non-leap years of 52 weeks, this means there are 71 leap weeks every 400 years.
Split into simpler sentences: Most leap week calendars take advantage of the fact that 400 Gregorian Calendar years have exactly 20,871 weeks. With non-leap years of 52 weeks, this means there are 71 leap weeks every 400 years.