I'm assuming you are talking about the FL Resident Seasonal Pass? It sounds to me like you can start using the pass on 8/20 given that 8/19 is a blackout date.
I went to the Disney.com website and copied the info below:
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A Seasonal Pass entitles you to admission* to:
Magic Kingdom® Park, Epcot®, Disney-MGM Studios, and Disney’s Animal Kingdom® Theme Park
Except the following dates:
December 20, 2003 - January 3, 2004
April 3, 2004 - April 15, 2004
June 12, 2004 - August 19, 2004
December 18, 2004 - January 1, 2005
*Except for activities/events separately priced.