It is a really good idea to tune your instrument EVERY time you sit down to play it!
All wooden, stringed instruments go out of tune over time. With changes in humidity, the wood expands and contracts which causes the instrument to be slightly larger or smaller. Those very small changes in length cause the strings to go out of tune. By tuning every time you play or practice you are teaching your ears to hear what is and isn't "in tune".
A good (and free meaning ad-supported) tuning app you can use on a smart phone is at: https://stonekick.com/tuner.html
I like this tuner app because it includes the number representing the octave of the note you are trying to tune to. If you try to tune to a D that is too high you'll end up breaking your string. Strings are designed to be able to tune to a note and the few notes lower and well as the few notes higher than that note but they can't be tuned to a note a whole octave above the note that the string is supposed to be tuned to.
"Normal" (if there is such a thing as normal) dulcimer tuning is :
D 3 --- on the bass string (farthest away from you and usually a “wound” string)
A 3 --- on the middle string (plain steel)
D 4 --- on the melody string (closest to you and also plain steel)
Other Ways to tune:
There are many other ways to tune your Dulcimer. Other tunings that are now or have been popular in the past are:
DAA - popular when playing old-time fiddle tunes (this was the "original" tuning for the dulcimer)
CGC - popular with people that like to sing with their dulcimer (most people can sing in the key of C but not int he Key of D!)
DF#A - popular with people arranging modern music for the dulcimer (gives you many more notes on the fretboard than DAD)
Troubleshooting Tuning Issues:
If your dulcimer won't stay in tune or if you can't tune it at all, first consider when the last time you changed your stings. If you can't remember changing them then now is a great time! See the Strings page for more information.
If on the other hand your strings are new-ish, look to see if you have small screws on your tuning pegs. If you do ---- very carefully and very slowly --- tighten those screws to see if the tuning pegs will then hold the string in tune. I mean very, very slowly! Think of it as tightening the chest strap on your grand-baby's car seat! You go as slow as you possible can making very small adjustments to those screws!
If you don't have machine pegs and have wooden pegs instead, you can try this technique which is demonstrated using a violin: