once every 2737 years
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The pyramids of Giza in Egypt are currently aligned in exactly the same manner as three of the planets in our solar system, Saturn, Venus, and Mercury are aligned as of 12/3/12, 18 days before the famous and popular 12/21/12 which is the end of the Mayan calendar.
(This has been calculated using specialized software like Stellarium).
The planetary alignment on December 3, 2012 with the pyramids of Giza, occurs only once every 2737 years.
For more interesting stuff about the conjunction of dates, stars and the ancient calendar, you can read a related story, “2112 Decoded” at World Mysteries.
Star in the Stone 
I love the thought of a good alignment as much as the next star gazer. That said – there is no possible way for this to occur. The computer generated model shows a horizontal alignment to the north west of the pyramids, in fact it will be vertical and to the south east. .
http://notestoponder.wordpress.com/2012/08/21/planetary-alignment-over-giza-not-so-fast/
Yours are also valid points and information to digest. Thanks for the additional info.
The alignment that fascinates me is the one with the constellation Sagittarius and the centre of the Milky Way
Just letting you know I nominated you for a Liebster award.