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Hey 
I don't think it's robust enough. 

https://www.hebcal.com/holidays/

Shavout marked as yom tov Jun 11-13.

However, at Jun 11 it returns "Erev Shavout" and while I'm looking for "Shavout" it was not found "Erev Shavout" (Jun 11) so my code regonize as business day while it's not. So I changed the code to looking for substring of "Shavout" which wasn't good as well since it's found "Shavout II" (Jun 13). 
"isHoliday" in my point of view will be only Erev Shavout and Shavout but I need some how to find these specfic two.