>>11683
Well, basically a calendar is a system used to measure time. I have the strong belief that besides community-calendars, individuals must forge their own calendars based on the weeks of their cycles, be they 3-day weeks or 13-day weeks.
That being said, since you are asking about ancient calendars: you could be interested in something called the Runic Calendar. It is based on a 19 year Metonic cycle, but I only started reading about this recently and don't understand it too well yet.
From the Ancient Calendar, there were many depending on the location. On Celtic calendars New year started after Samhain, whereas in Roman calendar the year began in Spring, and notation was used in a system a bit too complicated, which is why it was discarded (depending on the month you count the day before the Kalendas, or first day of the month, or the day before the Idus, or the 13/15 th of the month). Ancient Sumerian calendar was based on 360 days, and others use calendars of 280 days as that is the gestation period of the human babies in the womb.
As >>11685 has said, Pagan calendars had 13 months because they were lunar calendars. Varg Vikernes offers a Calendar on one of the MYFAROG pdfs: https://myfarogdotorg.files.wordpress.com/2013/01/the-myfarog-calendar.pdf
You can check also this site for info on some festivals, if that's how you intend to measure time: http://pagancalendar.co.uk/
Last but not least, I want to ask something I consider relevant to this thread: The marking of time. As you know, we mark this year as 2016 due to imposition by foreign forces. Some people propose the use of a Runic Era notation, so today it would be 2166 RE, but there are others (Ab Urbe Condita, Holocene Era…). Which one should we use, and why? The numbers we use are based on arabic symbols, yet there's no doubt ancient germanic peoples used numbers, but I haven't found how they marked numbers.