When we talk about the Mayan Calendar, it is better to talk about a calendaric system where you have several different techniques to date a day.
Mayans did not have a single calendar to use, but they characterized the date based on different kinds of calendars that they designed. They mostly focused on the Tzolkin, the divine calendar, where a year is equal to 260 days.
The civil calendar Haab was more likely to our calendars that we use today. Haab calculated a year as 365 days. Mayans used the Venus cycle as a kind of a sign to show them the right day to organise, and the Long Count is similar to the Julian Calendar.
Here is how a Mayan date looks like:
Lets say today is :24.11.2009:
The Mayan date eqivalent is 12 . 19 . 16 . 15 . 17, 12 Caban, 15 Ceh.
12 . 19 . 16 . 15 . 17 is the Long Count date.
12 Caban is the Tzolkin date.
15 Ceh is the Haab date.
This calendaric system that the Mayans founded and created is the outcome of strong observation, calculation and hard work.
Today we still don’t know how Mayans knew about the planets, the space and the stars and how they were so progresssed in astronomy.