Pi day's child

John says

It is cute but it's a bit like musicians' jokes - you have to be in the club already to appreciate them.

Except drummer jokes, which are essentially polish jokes or blonde jokes. easily translatable.

Carping note:

What else are blogs for?

Since PI is irrational, using an intrinsically rational thing like a date is odd: you might as well call it "three and seven fiftieths day".

Yes; however,if you celebrate it at 1.59 AM you can get a few more decimal points out of it, and if you allow nanoseconds there is presumably one instant you can converge on to as close an accuracy as your clocks allow.
However, it has admittedly been suggested that the rest of us non-Yanks celebrate on 22 July, which is an undeniable approximation.

Point of usage: if we non-US people refer to a day/date, aren't we just as likely to say, for example, March 16, as we are to say 16th March?

Yes, but it's the numbers that matter (though I suppose you could convey the concept graphically by making a circle with your thumb and forefinger and moving your finger in and out rhythmically to symbolise the radius; try this in a crowded bar and see how many mathematicians you pick up).