This calendar was devised by the elves centuries ago, and was adopted by humans after the Agricultural Calendar was seen to be less useful outside of planting/farming scenarios.
The Lunar calendar is currently the least used of the calendars among humans. Elves and nocturnal races still utilize the Lunar Calendar among their own groups, but those who travel the world learn the Solar Calendar as well.
This calendar is composed of 12 months, each 33 nights long, leaving 4 nights unaccounted for. The remaining 4 nights are collectively referred to as “no moon” or “black moon”. These 4 nights are believed to be an evil and chaotic time among the superstitious. Though they always fall between the end phase of one moon cycle and the start of the next, these 4 nights randomly shift each year. While it may fall between the 6th and 7th moon one year, the next year it may fall between the 1st and 2nd.
To remove the issue behind these erratic 4 nights and to allay the fears behind the superstitions, human scholars devised a simpler calendar which was inspired by the two calendars before it. This became the modern Solar Calendar.