"What does she do in the afternoon?"
Translation:Що вона робить удень?
They are basically always interchangeable in locative and accusitive. The surrounding letters/words will determine which is the better one to use. If you have a bunch of consonants, use "y", if there are vowels around, use "в".
You will also see this with some nouns (вчитель & учитель). Even Україна is sometomes rendered as Вкраїна in poetry.