A way to finesse "calling the hostess" is - if you know of other people who are invited - ask THEM what "White Cocktail Attire" means to them.
It's natural to be hesitant to call the hostess, but despite my pre-retirement business years and various positions I've never been invited to a "White Cocktail Attire" party... just tell her the same, that you've never been invited to that kind of party and that the Internet research you've done isn't definitive. She'll be glad you asked!
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