No he had several weeks. http://www.loc.gov/exhibits/gettysburg-address/ext/TI9214A.html

http://www.loc.gov/exhibits/gettysburg-address/ext/trans-formal.html

These are 2 letters sent by David Wells who was the primary organizer of the Cemetery and the Dedication, dated November 2nd, 1863, almost 3 weeks before the date.

I direct you especially to this quote which should make it clear the Presidents role.

It is the desire that, after the Oration, you, as Chief Executive of the Nation, formally set apart these grounds to their Sacred use by a few appropriate remarks.

Now what he was doing right up to the night before the day was editing his remarks.