> Since I am so good at maths, I have calculated that sometimes this year,
  > it's Emacs' 40th birthday! Can we make it even more specific with a date,
  > perhaps, so that those of us that wants to can celebrate with some cake? :)

I don't have any records of when the first Emacs was usable for editing.
It would have been some time in late 1976, I think.

But it may not have been an identifiable moment, anyway.  I probably
started editing with Emacs when it was not really ready to ask anyone
else to use.

One option is to imitate Christian practice by fictionally attributing
to this event a date that is celebrated for some other reason.  Thus,
we could celebrate Emacs's birth on Grav-Mass day, December 25.
(See stallman.org/grav-mass.html.)