The mayan calender is not a usual calender
It's a glyph, not a book.

4 concentric circles around a images of the seasons with what has been interpreted as a death head at the center.
the inner most circle represents the 240 ritual days in the mayan calender
the one beyond that is the 365 days of the solar cycle.
One track clockwise, the other counterclockwise.
Beyond that are the years in the years and beyond that are the "centuries"
Aztec centuries are 52 years long, hence the quotes.
The Calender also acted as a Almanac, showing what days were days the gods would be favorable and everything would go well, and what days when things would go horribly wrong.
So taking into account when Europeans discovered Mayans and figuring out what day the Mayan calender was on... the Europeans figured out the Mayan Calender would run out on December 21st, 2012.
On that day, the calender terminates and the only glyph relevant to time is the death head in the center.
On December 22nd 2012, the calender starts at day 1, year 1, century 1.
However the Mayan civilization collapsed do to European influence before it was known what happens on the 21st.
So people have been scaring themselves stupid thinking that the ancient Mayan gods will be extremely pissed off at the world and cause a major catastrophe on a biblical level.
and that's that story.