There are a lot of good dates from which to choose! (As related below, though, Tax Day is my favorite. That's also the one I've heard most often promoted as Buy a Gun day; since Congresscritters waste time and resources to declare things like National Pickle Day, perhaps some rep would be willing to officialize this one as well
But No, there's no "official" Buy a Gun day, at least nothing more official than International Talk like a Pirate Day.)
[For all of the dates below, please read as "the closest date on which stores are open, with a leaning toward days before rather than after."]
So, without getting *too* silly (Arbor day is good to buy a Garand, *and* plant the tree for a new stock!), I submit the following:
Tax Day is my favorite gun-buying day.
(And Paul Revere's ride of April 18/19 gets some credit here, too.)
Last Monday in May is Decoration Day / Memorial day -- a good tribute to those who've died in military service is to reduce the chance that this country will ever be attacked (at least by a conventional army)
Independence Day is a good choice, always!
Halloween? Sure -- tell 'em you're going to be a crazy militia member, I guess, or the always unpredictable American Citizen.
Guy Fawkes day (5th November) seems an excellent one as well.
In years where it applies, Election Day is certainly a good day to buy a gun. ("Don't blame me, I elected to buy a gun.")
Thanksgiving is not a bad one -- very appropriate, in fact, IMO.
December 7: U.S. soil invaded (however you feel about the acquisition of Hawaii)
Christmas (though I believe in no relgiion, I do enjoy getting and giving presents, whether on Christmas or any other occasion) is definitely a good time for buying guns for those you love.
New Year's Day: "A new year? A new gun!"