What is significant about December 21st 2012?

Answer 1

A bunch of prophecies claim that to be the end of the world.
Notably the Mayans

Answer 2

2012 is sometimes claimed to be a great year of spiritual transformation (or apocalypse). Many esoteric sources interpret the completion of the thirteenth B'ak'tun cycle in the Long Count of the Maya calendar (which occurs on December 21 by the most widely held correlation) to mean there will be a major change in world order.

Accordingly, several eclectic authors claim that a major, world-changing event will take place in 2012:

The 1995 book The Mayan Prophecies linked the Maya calendar with long-period sunspot cycles.
The book 2012: Mayan Year of Destiny claims the Maya may have been instructed in their wisdom by disembodied entities from Orion and the Pleiades. Contact was maintained through shamanic rituals conducted in accordance with the movements of planets and stars. However, some Mayan priests living and working in Guatemala assert that there is no legitimacy to this theory.
The 1997 book The Bible Code claims that, according to certain algorithms of the Bible code, a meteor, asteroid or comet will collide with the Earth.
The book The Nostradamus Code speaks of a series of natural disasters caused by a comet (possibly as above) that will allow the third Antichrist to disperse his troops around the globe under the guise of aid in preparation for a possible nuclear war, although in the strictest sense it is unspecific as to nuclear war or some other natural or man caused destruction.
The book The Orion Prophecy claims that the Earth's magnetic field will reverse.
The 2005 book Beyond 2012: Catastrophe or Ecstasy by Geoff Stray reviews several theories, prophecies and predictions concerning 2012 and finds where authors have used faulty information or have bent the truth to fit their theories.
The 2006 book 2012: The Return of Quetzalcoatl by Daniel Pinchbeck discusses theories of a possible global awakening to psychic connection by the year 2012, creating a noosphere.
The 2007 book Apocalypse 2012: A Scientific Investigation into Civilization's End by Lawrence Joseph does not make any specific new predictions, but it reviews a number of 2012 predictions made by various sources, and presents arguments for the possible existence of dangerous positive feedback loops involving solar storms, Earth's magnetic field, cosmic rays, hurricanes, global warming, earthquakes, and supervolcanoes that may be on the verge of erupting.
Other prophecies and apocalyptic writings and hypotheses for this year include:

Terence McKenna's mathematical novelty theory suggests a point of singularity in which humankind will go through a great shift in consciousness.
Some proponents of a peak oil catastrophe place major events in 2012. Richard C. Duncan's book The Peak of World Oil Production and the Road To The Olduvai Gorge claims that the Olduvai cliff will begin and permanent blackouts will occur worldwide. Several studies predict a peak in oil production in or around 2012. [13][14][15]
Some alien-enthusiasts (e.g. Riley Martin), along with some new-agers, believe 2012 to correspond approximately with the return of alien "watchers" or "caretakers" who might have helped the first human civilizations with developing their technology and may have been waiting for us to reach a higher level of technological and/or social advancement. Beliefs range from the extra-terrestrials having benevolent purposes — such as to help human society evolve — to malevolent purposes — such as enslavement of mankind and/or manipulation.
There is a Hindu following indicating the appearance of an Avatar (God in human form) with God-like powers who will herald a new age. A website to this effect appears at: End of the World 2012

Answer 3

It is just the end of a cycle on the Mayan calendar and means nothing at all really, just like 2000 on our calendar being the end of a decade, a century and a millenuium. There were dumb predictions for 1000 and for 2000 as well, and many dumb wold-be prophets did not even wait for 3 zero years.

Answer 4

It is the last day on the Mayan calendar and some believe it is the day the world will end.

Answer 5

That is the date where the Maya calendar ends and it is said to be the end of the world

Answer 6

Four shopping days before Chrsitmas.

Answer 7

Binary numbers.

Or the day the Olsen Twins finally . . .

Answer 8

last day on the Mayan calendar

Answer 9

Nothing. Just another ordinary day.

Answer 10

My daughter's 28th birthday?