How do you say Where did you go on holiday in Finnish?

Answer 1

That would be something like:

"Minne menit lomallasi?"

Or:

"Missä olit lomallasi?" Which literally means "Where were you on your holiday?"
We usually ask "Mitä teit lomalla(si)?" , "What did you do on your holiday?". But the answer to your question would be "Missä kävit lomalla?"

Answer 2

In Finland, it's more common to greet each other with the exact holidays. Like "Happy(/merry) Christmas". But if you want to use that one it's "Hyvää lomaa" or "Hyviä lomia".