United States: Pi Day celebrations have already begun around the United States.
Pi is a mathematical constant having value 3.141592 or (14/3) and it is celebrated to honor the discovery of the constant number.
This day is generally commemorated on March 14th. Since the value of pi is (14/3) the day is celebrated on 14th of the third month of every year. This is an official holiday in the United States. The idea of the pi day celebration is to draw attention for improving maths and science education in the US.
People usually celebrate the day by eating pies, reciting as much on the pi number sequence as possible, and talking about the significance of the mystifying number.
On this particular day, air-crafts having pi sign  take off to the skies to attempt to spell out the infinite pi sequence across 100 miles of sky in Austin.
Thus, sky gazers in Austin, Texas can notice something interesting with skywriting airplanes namely, a “pi in the sky” on this pi day.

Video on pi day celebrations