I’m busy taking the tests these days. And recently came across the following two tests:

(1) Where … on holidays?

(a) you go (b) do you go © do you going (d) are you go

Is the sentence itself correctly written? ---> Is there a phrase [b]on holidays[/b]?

(2) I worked on a farm … the holidays.

(a) for
(b) since
(c) while
(d) on

The answer key is b[/b].

I learnt from "English Grammar in Use" (R.Murphy) the following:

  We say: (be/go) [i]on holiday[/i]/[i]on business/on a trip[/i] etc.
  I'm going [b]on holiday[/b] next week.

  We can also say 'go to a place [b]for a[/b] holiday/[b]for my[/b] goliday(s):

   Steve has gone to France [b]for a holiday[/b].

   To sum up: Can we also say [i]on holidays[/i] Or [i]on the holidays[/i]?

   Many thanks.

(1) Where do you go on holidays? is correct. (The speaker means there are many holidays in a month/year, and he wants to know where you usually go on these days)

(2) I worked on a farm on the holidays. (Here it may be acceptable if the speaker means some particular holidays)

Also possible, and more natural than the options provided by the question:
‘I worked on a farm in the holidays.’
‘I worked at a farm in the holidays.’