I am planning a public entertainment event in the city of Atlanta.  I
need to pinpoint dates for this event, and want to be able to
cross-references the dates with a local and national events calender. 
I want to be able to avoid any conflicts with other major local and/or
national events that may compete for attention --- such as the Oscars,
or a big local Festival, etc..  Is there a website for event planners,
or other such online resource, that allows one to compare their event
against a calender of others?

Look at last year for that time period Time magazine, Tv guide, and
the local paper. That should help you.