If I remember rightly, the Mayan calendar is based around the planet Venus. It's quite a complicated cycle, but basically they believe that the current world age will come to an end on that date. I know the Pleiades are involved too... I have a feeling it's supposed to happen as the Pleiades are setting and Venus is rising.

It's been a long while since I read up on it though so I might be wrong, but I do know for sure it's connected with Venus.

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"The stars are my home"
"I've seen things you people wouldn't believe... Attack ships on fire off the shoulder of Orion... I've watched c-beams glitter in the dark, near the Tanhauser Gate... all those moments will be lost, in time... like tears in the rain..."