Yea I've never heard anyone say 記念日 in that phrase before... it had a few hits on google so I figured I'd try it.
After a year I remember my wife saying １年おめでとう... she's Japanese (but sometimes I'm not so sure about her grammar...
).
"Dunno... It must be my old school kinda thinking but I'd feel reluctant to say おめでとう when I myself represent 50% of the party involved. "
That's interesting. I thought that for a second but I wasn't so sure... I feel like people would say either one although I totally understand where you're coming from.
Yamagata sounds like a cool place though. I've always wanted to go there. I used to have a penpal from there and she taught me a little bit of 山形弁, which was very interesting to say the least. I wonder how the folks in Yamagata would say this.