Once a month (approximately) Google recalculates the Page-rank each site and, therefore, modifies the results of searches.

The 'Google Dance' is the period time between the beginning and the end of this update. Usually take about four days, during which time they get different results in each of Google's servers: www.google.com, www2.google.com, www3.google.com and www-fi.google.com.

The best time to drop pages on a website is during the 'Google Dance'. If left too long between the end of this update and the publication of new content reduces the number of pages to be included in the next update.

This event usually occur approximately once a month. Here a table of dates. To go looking for these days how it will change the position of your pages, in addition to this useful disponeis tool.

Tool 'Google Dance'

During the duration of the 'Google Dance' (as it updates the Page-rank of each page indexed by Google), you get different results in www.google.com servers, and www3.google.com www2.google.com

With this tool, we know the behavior of the 'Google Dance' (takes about 4 days) and what the next position after this message update.