Isn't the Quezalcoatl the serpent on the side of teh sun temple, and on the Summer Solstus(I think thats when) the sunlight makes its body.
I am fairly sure you are thinking of the Kukulcan pyramid at Chichen Itza. The shadows form two snake bodies that travel down the pyramid and adjoin
with two stone plumbed serpent heads at the foot of the pyramid. The plumbed serpent represents evil and the road to the underworld. Also I think it
is the winter solstice, not really sure on that though.