The 4-Week Leap Month Calendar has 13 months of exactly 4 weeks, i.e. 28 days, in common years and an additional 14th month of equal length in leap years. At 364 days, the common year is a bit too short to be an appropriate solar calendar and the months are also a bit too short to make it a good lunar calendar, but it’s a reasonable compromise of both with a focus on very regular months.