It may happen some day that you will have to mic up an accordion. Stranger things have happened. I can tell you from experience that working a stage at an accordion festival isn't even close to the top of the list of strange things you'll have to stick a mic on. So with this little bit of knowledge in your back pocket you'll be up and running in no time and you can polka till dawn!
Accordions come in all sizes and shapes, form a little concertina to a huge six reed dance hall model. Only the smallest can be effectively captured with one mic though. While a large diaphragm condenser might work for a solo performance, an accordion in the midst of a larger ensemble will do better with the isolation you can get from dynamic mics. Luckily the choice is easy, just grab two of anything and put one on the keyboard side and the other on the chord side. Think of it as a soprano voice and a baritone.
Blending the two mics together will really do the instrument some justice. Then again you might luck out and get a player with a pickup installed, or even one of those newfangled MIDI enabled accordions. (Yes... that's a thing.) You could even go so far as to throw some reverb on the keyboard side to make it sound a little more like an old polka record.
OK, I'll stop making fun of polka. The accordion can actually be found in lots of different musical styles, from Irish to Italian, and from Eastern Europe to Louisiana. Knowing how to treat one right will score you some points with the band.
Tune in tomorrow for another quick mic trick, and we'll be taking requests from the Brethren of the Knob and fader so get yours in.