I've always thought 'traditional' was much easier to 'define' than 'folk'. Traditional is what people do with it. You can't separate 'traditional' from its context. Audience expectation - thyat's why a thing can become 'traditional' after only a handful of occasions. (Or, as Terry Pratchett once said in a lecture to the Folklore Society, "I've come to the conclusion that 'traditional' is something you've already done once" - perhaps a little too far!). However:-
Is that particular song traditionally sung in that environment?
It is traditional to put up Christmas trees over Christmas.
It is traditional for the hunt to meet on Boxing Day.
Is Happy Birthday traditionally sung at birthdays?
Is it traditional to sing Flower of Scotland when Scotland playing in the 6 nations (sorry, U.S. - Annual Rugby Football round robin between England, Ireland, Scotland, Wales, France & Italy)?

Pray the thread doesn't drift into 'folk'!

Tom