My first post after I got married: From the thread “Led Zeppelin - reforming and touring - for real, this time” (Came 10 days after my wedding)
Pick a momentous moment, vanity search yourself (with results being displayed as posts to make it easier to find) and let’s see it.
I assume you’re asking for first posts after momentous events in your (the poster’s) own life, not first posts after momentous events in the world like the first moon landing.
If so, I would think the first post after a momentous event would be about that event and sooner than 10 days after!
Since late May, I’ve been posting descriptions of several events in my life that seem momentous to me, over in the General Aviation thread, generally the day afterward or even the same day.
http://boards.straightdope.com/sdmb/showthread.php?p=20241198#post20241198
http://boards.straightdope.com/sdmb/showthread.php?p=20273705#post20273705
http://boards.straightdope.com/sdmb/showthread.php?p=20306792#post20306792
http://boards.straightdope.com/sdmb/showthread.php?p=20339064#post20339064
And just today:
http://boards.straightdope.com/sdmb/showthread.php?p=20396258#post20396258
(These are all detailed posts about my recent adventures flying sailplanes. Read them if you think that sounds interesting.)
My first post after my mom died was a new thread about her death.
My first non-mom post after she died was expressing sympathy in joyfool’s thread on the loss of her husband.
My first non-death-related post after Mom died was in a West Wing thread.