In the irrational American way of denoting dates, today is 3-14-2012, which makes it Pi Day as the irrational, mathematical constant π=3.14… The suitability of 14 March as Pi Day is highly dependent on cultural context.
What π really represents is the ratio between the radius of an ideal circle and its circumference (C=2πr) or its area (A=πr2). We can determine the day of the year that best expresses this ratio if we treat the 365.25 days of the calendar year as either the circumference or area of the circle and calculating the day that would be the “radius”. In the case of the circumference, r~58 or 27 February. In the case of the area, r~11 or 11 January. Personally, I prefer to think of the year as the circumference, as it is linear and repeating, but you may pick your poison. I have both on my calendar and like to think of them in the same way as Catholic and Orthodox disagreements about the timing of Easter.
And, now is the point where I retract my earlier assertion that 16 February should be Phi Day. The more precise expression of the ratio gives us Day 226 (Φ~365/226) on the calendar, which is usually 14 August (except in the leap year). Please revise your calendar’s appropriately.