The 2012 phenomenon was a range of eschatological beliefs that cataclysmic or transformative events would occur on or around 21 December 2012. This date was regarded as the end-date of a 5,126-year-long cycle in the Mesoamerican Long Count calendar, and festivities took place on 21 December 2012 to commemorate the event in the countries that were part of the Maya civilization (Mexico, Belize, Guatemala, Honduras and El Salvador), with main events at Chichén Itzá in Mexico and Tikal in Guatemala.

Various astronomical alignments and numerological formulae were proposed for this date. A New Age interpretation held that the date marked the start of a period during which Earth and its inhabitants would undergo a positive physical or spiritual transformation, and that 21 December 2012 would mark the beginning of a new era. Others suggested that the date marked the end of the world or a similar catastrophe. Scenarios suggested for the end of the world included the arrival of the next solar maximum; an interaction between Earth and Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy; the Nibiru cataclysm, in which Earth would collide with a mythical planet called Nibiru; or even the heating of Earth's core.

Scholars from various disciplines quickly dismissed predictions of cataclysmic events as they arose. Mayan scholars stated that no classic Mayan accounts forecast impending doom, and the idea that the Long Count calendar ends in 2012 misrepresented Mayan history and culture. Astronomers rejected the various proposed doomsday scenarios as pseudoscience, having been refuted by elementary astronomical observations.

December 2012 marked the conclusion of a bʼakʼtun—a time period in the Mesoamerican Long Count calendar, used in Mesoamerica prior to the arrival of Europeans. Although the Long Count was most likely invented by the Olmec, it has become closely associated with the Maya civilization, whose classic period lasted from 250 to 900 AD. The writing system of the classic Maya has been substantially deciphered, meaning that a text corpus of their written and inscribed material has survived from before the Spanish conquest of Yucatán.

Unlike the 260-day tzolkʼin still used today among the Maya, the Long Count was linear rather than cyclical, and kept time roughly in units of 20: 20 days made a uinal, 18 uinals (360 days) made a tun, 20 tuns made a kʼatun, and 20 kʼatuns (144,000 days or roughly 394 years) made up a bʼakʼtun. Thus, the Maya date of 8.3.2.10.15 represents 8 bʼakʼtuns, 3 kʼatuns, 2 tuns, 10 uinals and 15 days.

There is a strong tradition of "world ages" in Maya literature, but the record has been distorted, leaving several possibilities open to interpretation. According to the Popol Vuh, a compilation of the creation accounts of the Kʼicheʼ Maya of the Colonial-era highlands, the current world is the fourth. The Popol Vuh describes the gods first creating three failed worlds, followed by a successful fourth world in which humanity was placed. In the Maya Long Count, the previous world ended after 13 bʼakʼtuns, or roughly 5,125 years. The Long Count's "zero date" was set at a point in the past marking the end of the third world and the beginning of the current one, which corresponds to 11 August 3114 BC in the proleptic Gregorian calendar. This means that the fourth world reached the end of its 13th bʼakʼtun, or Maya date 13.0.0.0.0, on 21 December 2012. In 1957, Mayanist and astronomer Maud Worcester Makemson wrote that "the completion of a Great Period of 13 bʼakʼtuns would have been of the utmost significance to the Maya." In 1966, Michael D. Coe wrote in The Maya that "there is a suggestion ... that Armageddon would overtake the degenerate peoples of the world and all creation on the final day of the 13th [bʼakʼtun]. Thus ... our present universe [would] be annihilated ... when the Great Cycle of the Long Count reaches completion."

Coe's interpretation was repeated by other scholars through the early 1990s. In contrast, later researchers said that, while the end of the 13th bʼakʼtun would perhaps be a cause for celebration, it did not mark the end of the calendar. "There is nothing in the Maya or Aztec or ancient Mesoamerican prophecy to suggest that they prophesied a sudden or major change of any sort in 2012," said Mayanist scholar Mark Van Stone. "The notion of a 'Great Cycle' coming to an end is completely a modern invention." In 1990, Mayanist scholars Linda Schele and David Freidel argued that the Maya "did not conceive this to be the end of creation, as many have suggested". Susan Milbrath, curator of Latin American Art and Archaeology at the Florida Museum of Natural History, stated that, "We have no record or knowledge that [the Maya] would think the world would come to an end" in 2012. Sandra Noble, executive director of the Foundation for the Advancement of Mesoamerican Studies, said, "For the ancient Maya, it was a huge celebration to make it to the end of a whole cycle," and, "The 2012 phenomenon is a complete fabrication and a chance for a lot of people to cash in." "There will be another cycle," said E. Wyllys Andrews V, director of the Tulane University Middle American Research Institute. "We know the Maya thought there was one before this, and that implies they were comfortable with the idea of another one after this." Commenting on the new calendar found at Xultún, one archaeologist said "The ancient Maya predicted the world would continue—that 7,000 years from now, things would be exactly like this. We keep looking for endings. The Maya were looking for a guarantee that nothing would change. It's an entirely different mindset."

Several prominent individuals representing Maya of Guatemala decried the suggestion that the world would end with the 13th bʼakʼtun. Ricardo Cajas, president of the Colectivo de Organizaciones Indígenas de Guatemala, said the date did not represent an end of humanity but that the new cycle "supposes changes in human consciousness". Martín Sacalxot, of the office of Guatemala's Human Rights Ombudsman (Procurador de los Derechos Humanos), said that the end of the calendar has nothing to do with the end of the world or the year 2012.