Turn them over and make new notches. If it uses a 0 fret just open the slots so he bass string fits will usually work since the nut is for lateral string positioning. Or, you could just play the bass string as the melody [DAd] like a friend did.
@richard-streib 
Any date that is usually mild in both hemispheres.
Length of day/night [equinox] is irrelevant and is assuming everyone can, or will, take time figure out the right date. It's easy to mark a consistent date, or day, on a calendar.
I'm really the right one to talk about dates [sarcastic], I usually remember about an event the next day or forget about them entirely. 
How about the last Saturday in March? It's not a normal work day and is early spring.
This could whole thing could result into some really interesting posts.
"I'm going to a Dulcimer Fest on ADD [Appalachian Dulcimer Day]. I'm taking a class from a instructor who wants everyone to bring a CAD [Chromatic Appalachian Dulcimer]. He says he uses a LD [Lap Dulcimer] with all the frets. I'm going to bring my matching MD [CMD] and a friend is going take their AD and tape on some toothpicks".
When I was introdued to the instrument, it was 'Mountain Dulcimer'. I still think of it that way [or MD]. The other names came later, in some cases, much later. Bill will always be be Bill, not William.
I kind of feel there is too much overthinking going on. Much of the time it is called an MD, or Dulcime[o]r on this site. On the old ED, MD, or Mountain Dulcimer vs HD, or Hammered Dulcimer. I think most general references in news articles, Mountain Dulcimer is most commonly used [probably wrong, but
].
If we commonly refer to it as a MD or Mountain Dulcimer internally [ FOTMD], why use a different name for a special day in it's honor?
Although it would probably be nicer to have it sometime during the late spring to early fall.
No on the tunings, yep on random, just pick any day, the 3rd friday of any month, 2nd blue moon, or any other random day. Don't bother trying to make some kind of association with the dulcimer, that's not relevant.
I feel associating a particular date with a primary purpose of celabrating the overall appreciation of the MD is kind of restrictive. Almost all of the posts refer to subsets of the main subject, the MD. Just picking a random date [the 164 day of the year] and calling it 'Dulcimer Appreciation Day' or something makes more overall sense. Then all of the things expressed are valid subjects for inclusion.
'Jean Ritchie Day'.
Why tie it to some specific dulcimer event which could be the name anyway? Like 'Jean Ritchie Day' or 'Kentucky Dulcimer Day', etc.
The 'why' the day was selected is irrevelant, just as long as a day is identified and observed consistently.
I suggest the same day as 'play music on the front porch day'.
Aug 1 is 'Uncommon instrument Awarement Day'. 
It could be that the MD is particularly resonant to B, which makes that note 'stand out' more. Since you are trying to play the tune in Aeolion [capo at 1] mode it may work to try tuning to DAc.
Sorry, I forgot to say to set the bridge to the same measurement as fret 1 to fret 8.
You could try tuning the bass string to C on the 7th fret, position a nut, then tune the melody open to the bass 7th.
Fixed 'do' and movable 'do'.
Fixed,'do' always = C.
Movable, 'do' depends on scale used.
To be clear; The notes for DAd are D3A3d4, much higher the strings break.
Another source of strings;
https://www.stringsbymail.com/
I've used both
Here is a short video about tempered tuning that may be of interest, especially to those of us in the learning mode. It was presented on another site [autoharp] and I'm 'borrowing' it.
A floating bridge is one that can be moved across the top of the fretboard. The other bridge style fits into a slot. Since you're asking I would think you're MD is set up for a floating bridge. Use your tuner to adjust it to the proper location. Tune the open note, check it at the 7th fret, if sharp, move the bridge away from the nut and repeat
It's not necessarily a kit. It may be a modified kit, made from scratch or with made from a variety of purchased parts.
I think I would check the tuning before doing anything. Check using the 0 fret as the starting point then using the nut as the starting point. The frets should only be correct using one or the other. There is a significant difference in string length there. It may have been set up to use a slide, as in dobro style
Lower left corner of the post is a thumbs up icon.
I always carry a pencil, and paper is usually available, and do a rough sketch [for a lot of things]. I would say to most folks that it sounds something like the more treble side of an acoustic guitar [sorry about that ] with less volume and sustain, because that's probably what most can relate to.
It's interesting to see that many folks have more instruments than just mountain dulcimers.
In addition to my dulcimers, I have a bodhran, half a dozen or so diatonic harmonicas, a chromatic harmonica, several pennywhistles, a full size keyboard, and 5 autoharps [three chromatics, two 2 key diatonics, FC and GD] and a 15/16 hammered dulcimer I made and still have. I've tried guitars and a banjo but they didn't, physically, work for me.
I guess I have IAD, but music is such a fascinating subject and has so many facets and challenges it's easy to overindulge! Especially for a non-music oriented retiree [no music instruments at all before retiring].
A piece of wire [paper clip] or wood [tooth pick] or ---- taped in place [tape parallel to the strings] in one of the wide spaces. Frets are usually approximately .040" -.045" or so high. Use your tuner to position the temp fret for the appropriate note.
Multiple dulcimers and their use as tools are really good points.
I should add that of the 2 the I use the most, one is fully chromatic, mostly for practice, and one is modern diatonic [1+, 6+, 8+, and 13 +], practice and with others. My bass is fully chromatic and the rest split between the two.
Thinking about the basic question, I suppose that the fact I have, and use, extra frets can be seen that I do feel limited with a pure diatonic setup. That even extends to the harmonicas I mess around with as I have both, chromatic and diatonic.
There are a few more things that can help; temporary frets, and 4 equidistant string/chromatic tunings [ie. DAA#D]. I choose to go full chromatic which allows me to have the accidentals when needed without changing the MD sound. But I don't do noter/drone style playing either, the extra frets, without being extra careful with noter pressure, put a 'bump' in the music flow when N/D playing.
Eleven: Folkcraft Resonator, 5 string Blue Lion Jam strung as a 3 string bass, Bonnie Caroll, Black Rose, McSpadden Kit [my first MD], six I made, including one made from Koa, and a travel MD only 4" wide, a Banjimer, and a TMB that I later modified into a double bass using bass guitar tuners and strings.
The Folkcraft, Blue Lion, travel, and the Koa are the ones I use the most.
It seem to me the 'when' needs a lot emphasis, like a larger text font, in color, no abbreviations and possibly other attention gathering techniques. It may help to include a link to a time conversion site. The 'when' is probably the most important piece of information I use to plan things.
Derik, you need to start at the third fret to get the scale you are expecting. The reason is the frets [excluding any half or plus frets] are laid out in mode patterns. In other words, each set of 8 consecutive frets [including the nut/0 fret] form a series of tones/semi tones [steps half steps]. This allows or supports. listing the eight notes in an octave to begin with each of the the notes in turn, eg., ABC#DEF#G [mixolydian pattern] or DEF#GABC# [ionian pattern], etc, each series being a different tone/semi tone order.
The most critical thing is getting their attention in the first place.
Dusty;
I kinda think you identified one of the primary sources of beginners in your original post, current dulcimer players interacting with potential players. It seems there is a need to motivate us to point new folks to appropriate 'sites. 
1. Glaringly obvious advertisement/schedule on a club home page.
2. I believe there is some way to boost internet search hits on the 'net.
3 Maybe Strumelia can come up with a way to advertise beginner classes, maybe a group or forum? I'm not sure the 'beginners group' would work very well because all posts move along in time, and sticky's [if made available] would eventually overload it. Maybe a new tab, 'Classes' at the top of the Home page, where the listing is for forums, members, rules, etc.? Inside could be all the different 'flavors: club, individuals. free, paid, beginners, advanced, n/d, finger,pick, flat pick, genres, etc.
Which brings up the existential question... if one were to put a fret marker on every fret- what purpose would it serve?
Could be both decorative and/or useful, depending on the design. 
I came to the dulcimer without any music background at all. Fret markers were just a decoration and my 1st dulcimer was a kit without markers. I didn't realize they had a purpose until I went to full chromatic fretboards. I looked at the 3-5-7-10 but they didn't make any sense to me [still don' t actually] so went to marking the diatonic frets since that layout was familiar. I suppose, considering they are really just location identifiers, that it's what a person gets used to using [without even thinking about them].
I just looked at my Folkcraft, it's marked at the 3-7-10. Didn't realize that before.
Inversion of Gm7 according to https://www.scales-chords.com/chord/piano/Bbgf
The difference is only a few thousandths of an inch. I suspect any difference in tone would be difficult to detect even with top notch equipment.
I've never seen one of the old ones. A call to Jim Woods at McSpadden may be helpful.
I installed one on a similar instrument. The one on the scroll was on the bottom of the scroll with the screw pointed more or less towards the nut. The bottom edge of the button was just above the scroll/bottom corner. There are 2 options for the rear one, on the side of the tailblock facing the listener, the other option is more difficult, re-position or share the anchors, and place the button between the anchors.
Where the wire hole is depends on the style of bar pickup. There are 2 styles I've used, one is aimed at violins with the wire exiting from the center of the bar, the other is the more common bar with the wire exiting from the end. Both are a bit longer than most fretboard widths, about 1 1/2 -1 5/8 I think. There may be shorter ones available. The end exit type can be carefully clipped shorter. The active elements are seen as small bumps in the skin, the cut S/B between the last 2 bumps, there 4. A good set of side cutters works. The hole on the end exit style is drilled right next to the fretboard and the wire/bar end covered with a lightly glued cap.
There are some that already have the bridge piece glued to the pickup element. I've not used any. I make the bridge to fit.
The wire can be fished out the jack hole with a piece of cutoff melody string with a hook bend on the end.
Use a pencil type soldering iron with a non corrosive, electronics flux core. Apply heat to the jack tabs to melt the solder not the wire.
