I sometimes say, “Another mile on the odometer”.
That seems to imply to me that there’s only ever one of them; care to explain the history of this fascinating family phenomenon?
I like the answers so far, thanks all!
Leaving the comfort of the womb to inevitably encounter taxes and death Day. :eek:
What - you wanted snappier? 
Celebrating the last time you touched your mom’s coochie.*
*Not applicable in West Virginia, Alabama, and Southern California
It may be that dad was feeling old and contemplating his mortality, but we don’t know for sure. That’s why I used the word “unknown” in my post.
As appropriate I pick from this list:
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
Sunday
So next week at work on Mark’s birthday I will say, “Happy Tuesday Mark.”
And I will mean it.
Anniversary of one’s World Debut.
Coming-Out Day
Spawning Day
Emergence Day
Obligatory Sex Day
Last February, I celebrated my 9th (anniversary of my 40th birthday).
“Expulsion from your mom’s vagina Day!”
The exclamation point is what makes it, of course.