Google Dance is an out-dated slang term used to describe the period of time in which Google used to rebuild its rankings, and as a result of these rebuilding, rankings of Web sites on Google's SERP fluctuate widely for a 3 to 5 day period. The "Google Dance" would happen anywhere up to 36 times per year.
i think as per my experience it happens generally once in a month and during that u can face drastically drop in your keyword position and after 3-4 days u will retain your keyword position back
Google dance
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