I reckon that the Mayans knew about numbers and knew about time. Thus they knew that they would have to stop writing their calender at some point or someone would be writing it for a very long time! So, they used the best reminder that they could as the last day of 'the calender'! And yeah, I expect they would have had a massive party if they had made it as a civilization up until now! Who knows what they would have achieved? Perhaps they got fed up and went to another planet in their space craft?
Peace
JahGuide