When the Friday before a weekend or the Monday after a weekend is a holiday, we call the resulting 3 days off work a long weekend.
Monday, September 17th, is Respect-for-the-Aged Day. Also, Sunday, September 23rd, is the Autumnal Equinox [one of two days every year when night & day are exactly the same length. The other day is the Spring Equinox]. Holding National holidays on Sundays doesn't have any meaning so Monday, September 24th, has been declared a holiday instead.
Wow! 2 long weekends in a row! Have fun!!!