A National Day of Reason, a secular contrast to the National Day of Prayer, I admit gives me mixed feelings. There are two reasons for this.
The first reason is simply that a National Day of Reason strikes me as a sort of me-tooism which some excitable - and more polite, I should add - members of the secular community go through every now and again. (A similar example is the 'tree of reason' idea - a tree with the names of scientific discoveries stuck on it, to be placed at town halls as a complement for menorahs and Christmas trees and so on.) The rationale is understandable but silly to anyone on a moment's reflection.
I don't imagine many atheists - or laymen with a science fetish - would propose a National Day of Reason if there wasn't a National Day of Prayer sitting around. To put it on the same day as the National Day of Prayer means that reason isn't even the focus. If it were in mid-June I'd see the point to it. As it stands, it is me-tooism, plain and simple.
The second reason the National Day of Reason gives me mixed feelings is that I am annoyed by the deification of reason and rationality among secularists. Please do not misunderstand me: Reason is a wonderful thing and one of the most useful tools we have as humans. My objection is that reason without sufficient data ranges from the interesting but unprovable to the useless to the dangerous. I am entirely in favour of reason, as I am sure most people are; however, if you're going to deify reason in opposition to faith, it should be as a trinity with empiricism and skepticism.
It is not (just) that religions are often irrational which means that they are not true; it is that there is insufficient empirical evidence to suggest that religions are true.
 Some would, surely. But some people are strange.
 The charge of 'that is irrational' carries as much weight and venom among some atheists as 'that is immoral' does among many Christians.
 Which is, to put it mildly, a terrible way of verifying truth claims.