The unofficial day to celebrate "bosses" is spelled "Boss's Day" or alternately as "Bosses Day" because it does refer to more than one boss.It is celebrated in the US and Canada on October 16.
Well, personally, I think that you should use "Boss's Day," one, because that is how it is spelled on my calendar, and two, because it is not necessary to use that extra "e," when you don't really...
That is the correct spelling of Boss's Day, although in common use you can see Boss' Day, which is technically improper, and Bosses Day, and National Boss Day for the apostrophe-challenged.