Tau Day or Two Pi Day
Tau Day or Two Pi Day is held on June 28. This event in the third decade of the month June is annual. 
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Two Pi Day, also known as Tau Day for the mathematical constant Tau, is observed on June 28 (6/28 in the month/day format).
June 28 is "Two Pi Day", also known as "Tau Day". 2π, also known by the Greek letter tau (τ) is a common multiple in mathematical formulae.
Some have argued that τ is the more fundamental constant, and that Tau Day should be celebrated instead.
Celebrations of this date jokingly suggest eating "twice the pie".
Similar holidays and events, festivals and interesting facts
Sunglasses Day on June 27
International Body Piercing Day on June 28
Paul Bunyan Day on June 28
Log Cabin Day on June 29 (was originated by the Log Cabin Society in corroboration with the Bad Axe Historical society);
Waffle Iron Day on June 29
Social Media Day on June 30
Please Take My Children To Work Day on June 30 (Last Monday of June);
Meteor Watch Day on June 30