You seem to be concerned with needlessly downloading data when it is, in fact, duplicate data on a "down day".

Why not use a canary index to check for down days? For example, before downloading data for all of your tickers, perform a quick check on the DJIA -- if the DJIA has not been updated, then it's a fair bet none of the tickers will have been updated either.

That way you eliminate the need for figuring out holidays. You script is adaptive and not wasteful of resources.

Matt