Answer:
"Duration" means length. A holiday duration means the length of time set aside for celebrating that holiday, which usually includes not having to go to school or do work. Many holidays in the US are only one-day holidays, but have been fixed to fall on a Friday or a Monday, so that people have what's called a "long" weekend.
Others are longer in length, or duration, such as "Spring Break" for school students, or the Christmas holidays.
Different customs in different countries lead to a wide variety of holiday durations.