"When did you feel happy?"
Translation:Коли тобі було весело?
Коли тобі було весело? = When did you have fun?
Коли ти почувався щасливим? = When did you feel happy?
I got desperate and tried the literal translation. Didn`t accept it. However, there is nothing wrong in Коли ти відчував себе щасливим?
This program sucks. Everybody here has complained that happy is not весело but щасливий/ва. Yet no one at Duolingo seems to be paying attention. Бути весело means to have fun, have a good time. To be happy is an internal state, щасливий.