The word 'Holiday' is widely used in British English, but 'Vacation' is in American English. But there's no difference in meaning. But, 'A Holiday' also means a single-holiday.

'On holiday' is standard British English. 'On vacation' is standard American English. There is no difference in meaning.

I would argue, Rosa Maria, that there is not a big difference between these two except perhaps the place and length of time one might assume with each. on holiday, to me, means short break from work (or school) perhaps because of a celebration (e.g., Christmas) with little traveling plans. on vacation, to me, means a long(er) break with plans to travel. But this is just hearsay and I would urge you to think of them as having the same meaning. Nick

Thanks a lot. It's very helpful!