I have three questions:
a) Can I use phrasal verbs in formal letters???
b) "On holidays my friends and I went riding". I saw in a book that this sentence is wrong. The correct form would be: "In the holidays, my friends and I went riding". Then I wonder to know why? I've looked the words "holiday" and "vacation" up in dictionaries but the only preposition I see is "on".
c) When I wish: "Have a nice vacation / holiday", I don't need to use it in the plural, do I?
You can use phrasal verbs, but you shouldn't use too many in formal writing. If there is an easy replacement, then it's a good idea to replace.
Without the definite article, we use 'on', bit with the definite article, we use 'in/during the holidays'
We use the plural for holidays for everybody:
I went on holiday to Spain.
What did you do during the Easter holidays?