May Day
Change to how Google assessed long-tail queries that cut traffic to thin, template-generated category and tag pages.
What changed
Around the turn of April and May 2010 sites with large numbers of thin, automatically generated pages lost traffic on long-tail queries while head-term rankings were largely unaffected. Google confirmed it was an algorithmic change to long-tail results and framed it as a quality judgement rather than a penalty. It is often read as a precursor to Panda. Only month-level precision is documented, so the start date is set to the first of the month.
Timing
Rollout began 1 May 2010. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.