From the Smithsonian National Museum of the American Indian: "December 21, 2012 marked the end of an important cycle in the Maya Long Count calendar. This cycle is composed of 13 periods, called baktun, of 144,000 days each. This 13-baktun cycle began on the Long Count calendar date 13.0.0.0.0 4 Ajaw 8 Kumk’u, and spans 5,125.366 solar years." 

"Monument 6 [shown here], from the archaeological site of Tortuguero in Tabasco, México, records the only known Maya inscription of the end date of the 13-baktun cycle. This end date, 13.0.0.0.0 4 Ajaw 3 Kank’in, corresponds to December 21, 2012. There is no evidence in these inscriptions, or in any other record, that the ancient Maya thought that the Long Count calendar would imply some kind of catastrophic “end.” These predictions were unfounded and are not shared by the Maya people."