As to name, I do not feel strongly about 'American dulcimer,' and Strumelia is correct that it's really too broad. I'm good with either 'mountain' or 'Appalachian' (regardless of pronunciation, lol). I think the terms 'lap' and 'fretted,' while accurate, are dry and descriptive, while 'mountain' and 'Appalachian' are evocative and descriptive.
I was feeling enthusiastic about MD and the banjo being the only two instruments indigenous to the US. I was also thinking of the large contingent of folks playing the 'Persian dulcimer' (lol) who might be inclined to say "Hey what about us?" so we need to be specific - and it seems like folks were thinking that way anyway.
Even so, many clubs and the Dulcimer Players News itself include both dulcimers, and I can well imagine some hammer-folks saying "Hey it should be about both dulcimers!" So, at least some of us will have to answer not just the question of 'why-that-date?' but also 'why-weren't-we-included?' To that, the native roots of our dulcimer seems a diplomatic and compelling comment. More compelling than "because the idea started on FotMD"! More diplomatic than "b/c you drown us out," lol.
Now, as to date: Speaking to Dusty's concern, it often happens that 'celebrations' for an important date are moved to the closest weekend. So there's always the possibility of a set date falling where it might, but the subsequent weekend being the time for a local festival or jam or a special Zoom thing or etc. [Note this raises the separate question of what exactly to do to mark it. I say best to leave that to local communities and whatever folks want to make of it. But that thought argues for something in nice weather - versus, say, Dec 12, which I otherwise like.]
As for timing it to something less specific/more general - eg, first Saturday of Spring - I think that's still easy enough to remember, and let's look at the specifics: First, 'vernal equinox' = spring equinox = first day of spring, and it is virtually always on March 20, 21, or 22 (Farmer's Almanac, almanac.com). This year it's Sunday, 3/20, and MD-Day would be the next Saturday, March 26. Similarly, it's usually going to be the last Saturday of March - another easy way to remember it 'roughly' and then be able to look up. Next year, 2023 (a possible 'first annual'), it would be Saturday, March 25.
Did you know that April 8 is National Banjo Day?? Rather nice that it would be close!
Oh, but would we have some other competition in late March?? Well, surprise or not, yes - but not significant: 3/26 is National Nougat and National Spinach Day. March 26 is National Lobster Newburg Day, Natl Medal of Honor Day, Natl Tolkien Reading Day and Natl Manatee Appreciation Day. So we can celebrate by taking a manatee out for lobster Newburg...
Somehow I think we're okay with regard to that!! And obviously there are National Days for almost anything, so it's high time we get one for the mountain dulcimer!!