The Hermatic Leap Week Calendar is a leap week calendar with 71 leap weeks per 400 years so having a mean year of 365.2425 days equal to the mean year of the Gregorian calendar.
The distinctive feature is that the years are grouped into hexades of six or five years. The third year of each hexade is its only year with a leap week. The length of hexade is determined by the last two digits of its first year's number.
A hexade is short (has five years) if and only if the last two digits of the product of 71 and the last two digits of the first year of the hexade is less than 26. Also a hexade begins at year 1, which began on Monday 25 December 1 BC in the proleptic Gregorian calendar.
This results in the northern winter solstice always occurring in or near the last week of the year.