<Ul> <Li> November 21, 2010 </Li> <Li> August 20, 2013 </Li> <Li> May 21, 2016 </Li> <Li> May 18, 2019 </Li> <Li> August 22, 2021 </Li> </Ul> <Li> November 21, 2010 </Li> <Li> August 20, 2013 </Li> <Li> May 21, 2016 </Li>

When does a blue moon happen in nature