In a bit of scientific silliness reminiscent of meter vs metre spelling arguments, proponents celebrate "tau day" Tau is 2*pi, hence the date, June 28. They claim tau is "more natural" than pi.
http://www.bbc.co.uk/news/science-environment-13906169 Since pi and tau are both irrational, I hope the celebration was concluded by 2011-06-28T07:38:41.1Z However, two pies would be perfect for celebrating two pi day.
