in holiday 泛指放假,但说话人说话的时候并不在放假.
如,i'd like to stay with my families in holiday.
in the holiday 指特定的假期,根据上文的延续.
如,Christmas is a spcial holiday in western countries.In the holiday,people usually stay with their families.
on holiday 处于放假的状态
如,i'm on holiday now.类似有on duty 上班中
另外补充一下,如果你前面曾经提起过某个节日,而现在针对这个节日说on,就要用on the holiday,特指在这个节日.否则,用on holiday.