in holiday 泛指放假，但說話人說話的時候并不在放假。
如，i'd like to stay with my families in holiday.

in the holiday  指特定的假期，根據(jù)上文的延續(xù)。
如，Christmas is a spcial holiday in western countries. In the holiday, people usually stay with their families.

on holiday 處于放假的狀態(tài)
如，i'm on holiday now.類似有on duty 上班中 

另外補充一下，如果你前面曾經(jīng)提起過某個節(jié)日，而現(xiàn)在針對這個節(jié)日說on，就要用on the holiday,特指在這個節(jié)日。否則，用on holiday.