I have done a lot of thinking about the answer and i cannot agree with what is said above.
There are no little seasons in the world. No one ever talks about days being longer or shorter. This means one thing, the planet does not have an angular tilt.
Without an angular tilt boy scouts could not tell the seasons. it would take high powered observatories to notice the difference in orbital position.
I am not claiming that maistors could not figure this out, I am saying that there is no way all society built their calendar around such a triviality.
Looking at human societies that did not use the motion of the sun to measure a year, the fall back is the moon. As far as i can tell a moon exists. There seems to be a region called "Mountains of the Moon" as at least 1 proof.
So the phases of the moon must be measured and however they are comes to a convenient year matching ours.