The Union Base Ball League has another meeting in New York next Tuesday.
-Cleveland Herald, September 21, 1883
A meeting of the Union Base Ball Association will be held at the Bingham House on Saturday next for the purpose of taking action on application for membership.
-The North American, October 17, 1883
Harold Seymour writes in Baseball: The Early Years that the Union Association was founded on September 12, 1883 and the article that I posted yesterday supports his claim. David Nemec, however, in The Beer & Whiskey League states that the UA was essentially put together in November of 1883 and its first meeting was on December 18. I don't believe that these two views are mutually exclusive.
The earliest evidence that I've seen regarding the organization of the UA dates to the middle of September, 1883 and, as I said, this supports Seymour's claim, which is repeated by Jon David Cash in Before They Were Cardinals. But while the original plan for the league may have been put in place in September, it's obvious that the formation of the league was an ongoing process and wasn't finished for some months.