>Is it observed by everybody in Japan?  (All 
shops, businesses, etc?)

No,it isn't. Basically Golden week sarted from
today in my country this year.(30th Apr to 2nd 
May were not holidays). Actually many people don't
work 30th to 2nd for their long vacation, but not
all of them. Maybe your seller will be taking 
long vacation. I think they will contact you on
next Tuesday or later.