Jon, I think it could be done, if one would admit a loose definition, as your word "pertains" implies. You would admit
(1) songs explicitely mentioning a date,
(2) songs mentioning an event that has a specific date (a battle, e.g.) even if the songs do not mention the date (if you scan through the database you see that quite often a specific date is given in the notes, and not in the song itself)
(3) songs about an event stretching in time (a war, a strike); the date for this song to be entered into the calendar would then be the beginning of a war, the end of a strike, the start of peace talks or so.

If only songs of type (1) were admitted, my guess is that a couple of days will be missing (I agree, however, with Bill: any possible date has most certainly been covered already, the problem is "only" the locating of the songs; to find up to 2000 songs mnetioning dates is not easy; of course, you could challenge some songwriters; I liked this idea).

There once was a folk music calendar in Germany with a song a day. But they had only about 10% of all songs fitting to the respective day, the other ones were just unrelated additions.

If it is too difficult to compile a calendar with the above three definitions, also admit release dates of records (any song on this record would qualify), birth and death dates of musicians (any song qualifies), dates of concerts ("July 26th, 1989: Christy Moore sings "Nancy Spain" in...").

I'd hate to compile such a calendar, but I'd love to buy it.
Wolfgang