I want to put a variation of the following just above a company description, but I'm wondering which preposition I should use:
Business Founded On December 2011
Business Founded In December 2011
Continuing my proud tradition of taking credit for something that FumbleFingers already answered in the comments...
Short Version: In would be appropriate here, On would be appropriate if you were specifying an exact day.
Long version: Using these definitions...
In - used to indicate inclusion within space, a place, or limits
On - so as to be attached to or unified with
...we can think of specifying month only as telling us the founding date is including within the limits of the month, while specifying a day tells us that the founding date is attached to that specific day.