Well, the debate may have finally been put to rest. Various pages on UO.com listed September 27, 2006 and, as the start date of Ultima Online, but apparently, that is not the case. According to Wilki, in this Stratics post:

Beta ended on September 23rd. While most Charter Edition players who pre-ordered and opted for overnight shipping didn’t get their box until the 25th, the servers were up on September 24th with a few players who obtained the game early playing, so that is the official launch date.

Wait, there’s more
UO.com - A Brief Retrospective (Sep 22, 1999) - SunSword comments

As many of you know, September 24th marks the second birthday of Ultima Online. This is a significant milestone in many aspects, perhaps not the least of which is the affirmation of the “100k+ Player Massively Multiplayer Online Game” as a viable long-term enterprise. The UO community continues to grow, and even as we approach the third year of service, our players have much to look forward to.

I remember taking off the 26th, which was a friday (I just checked again, it’s been a long time), to do some things, and actually received my Charter Edition that day and was able to login, but I had heard that the 27th was considered to be the first day. I wasn’t expecting it on the 26th (actually I can’t remember when I was expecting, just that I was expecting it before the end of the month).

Regardless, this week is the 9th Anniversary of Ultima Online :greatgift: :toot: and now we know the rest of the story.