Cuban π Day Flag

Puerto Rican π Day Flag

Panama π Day Flag

Portugal π Day Flag

Lower Case π Day Greek Number Flag

Greeks used letters to represent numbers as well.  In this case Gamma represent 3, Alpha represents 1, and Delta represents 4.  Reason being, Γis the 3rd letter, α is the 1st letter, and δ is the fourth letter in the Greek Alphabet.

Japanese Pi Flag

Putting the Pisces in π Day Flag