The site of the calendar was first excavated by the National Trust for Scotland in 2004. The calendar comprises a set of 12 pits, each likely to have contained a wooden post with one for each month of the year. The monument was used to chart the phases of the moon in order to track lunar months. To keep the time of the seasons but account for differences between the lunar and solar years, the sequence could be calibrated annually on the December Solstice sunrise when the site aligned. It is thought the calendar was used by hunter-gather societies to track the seasons so they would know when migrating animals were due to pass close by.
You can learn more in article in the University of Birmingham web site at http://www.birmingham.ac.uk/news/latest/2013/07/15jul-beginning-of-time.aspx and in the video below: