Without a telegraph connection it took a long time for news to travel 150 years ago. I did not see an “extra”; there was not much accurate news about the Battle of Antietam in the Dispatch for a few days after it was fought. There seems to have been quite a few rumors about the battle both North and South.
From the Richmond Daily Dispatch September 18, 1862:
Thanksgiving day.
To-day being the day appointed for Thanksgiving by the President, no paper will be issued from this office to-morrow. Should any news of interest transpire it will be issued in an extra.