"Коли ти почувався щасливим?" IMHO this translation is much better appropriate for this sentence.

This is a question for the psychiatrist.

Isn't "весело" rather "cheerful" than "happy"?

"Коли тобі було весело?" actually means "When were you cheerful/happy?"

For a physician to ask a patient a question to gage their mental health would be more like this:

"Коли був останний раз ви були щасливі/веселі?" ("When was the last time you were happy/cheerful?")

Or, "Коли було останний раз ви відчувалися щасливим/веселим?" ("When was the last time you felt happy/cheerful?"

'Веселий (m)/ весела (f)/ веселе (n)', 'веселі' (pl.) can mean: 'happy', "cheerful', 'funny'.

Happy = щасливий; веселий. Cheerful = веселий Joyful= радісний Fortunate = щасливий.

Why було?