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
帮你造了几句话,你可以通过理解句意来看区别,希望对你有用
on the holiday:
A few more traveller's cheques may come in handy on the holiday.
多带几张旅行支票，度假时会有用的。
in the holiday:
we (enjoy ourselves) in the holidays.
In the holiday season one finds the trains unpleasantly full of people.
节日期间，人们可以看到火车上拥挤不堪，人满为患。
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.望采纳，谢谢。
追问那么 for a holiday与这两个的区别是什么