What is the Google Dance?
Once a month, and totally unannounced,  Google has a major shift in it's rankings.  This is when Google "tweaks" is algorithm, and when it updates each sites PageRank and Back Links.

During the month there will be minor changes in rankings.  This is called 'Everflux'.  But only about once per month does Google Dance, updating the back links and the PageRank.  The dance usually lasts about 3-5 days.  During these days the Google Results will vary widely.

The Google Spider is called Googlebot.  Most sites are revisited by Googlebot only around the Google Dance time.  

Here is how Googlebot works.  The first time it finds your site it may visit it every few days to see if the content changes.  The theory being that if your site is constantly changing, then the Googlebot will continue to return on a daily basis.  But, if the Google bot finds your site and indexes it, and then returns to the site every week and the site has not changed then Google will be apt to put you on the "monthly dance" schedule.

Google doesn't announce dancing days, it just something that you have to "stay-tuned" to.  To find if Google is Dancing use the tool below. Click on "Search All Servers Now".  When any of the backlinks to Yahoo are showing different numbers than each other, then Google is Dancing.

Google is ONLY dancing when these numbers are different on any one of the www, www2, www3, or wwwde.   

The Google Dance usually goes on for 2-5 days.  During these days results on Google tend to change by the minute.  After the Google Dance is over each site will have been adjusted for PageRank and Back Links.

Dates that the Google dance started:

June 15, 2003

Google's Data Centers

For every domain www-xx.google.com,
there is an additional domain www-xx2.google.com.

Another Google Dance viewer by MarkHorrell.com

Google Deepbot - June News: Deepbot may be "dead" now as Freshbot seems to be Deepbot now:

What was Deepbot?
About 5-10 days after the Google Dance has "settled" Google sends out another set of spiders which have been named "Deepbot".  The Deepbot spiders IP address starts with 216. (as opposed to the regular Google spider that starts with 64.).

The Google Deepbot usually starts to come around a few days after the Google dance has "settled down".  This is when Google may index several of your pages.  Your link structure and PageRank are two of the items that Google uses in determining "how deep" they will index your site (how many pages they will record).

Past DeepBot dates:

April 15, 2003 March 10, 2003 February 6, 2003 January 3, 2003 December 2, 2002 November 3, 2002 October 3, 2002

To follow the beginning, ending, and new theories on changes I prefer to go to Webmaster World and read the forum on  Google news.  Each month there is an extensive thread on the Google Dance, and then after the Google Dance there is a thread with new theories on new changes in the algorithm.

More info on Deepbot and Freshbot.  More info on Everflux

Another tool for viewing the Google Dance is HERE