Approximately once a month, Google update their index by recalculating the Pageranks of each of the web pages that they have crawled. The period during the update is known as the Google dance.
Because of the nature of PageRank, the calculations need to be performed about 40 times and, because the index is so large, the calculations take several days to complete. During this period, the search results fluctuate; sometimes minute-by minute. It is because of these fluctuations that the term, Google Dance, was coined. The dance usually takes place sometime during the last third of each month.
Google has two other servers that can be used for searching. The search results on them also change during the monthly update and they are part of the Google dance.
For the rest of the month, fluctuations sometimes occur in the search results, but they should not be confused with the actual dance. They are due to Google's fresh crawl and to what is known "Everflux". Resources - webworkshop.net