"Where are you on Tuesdays at 8:30 in the morning?"
Let me break it down:
* donde - where
* estas - are you (informal, singular "you")
* los martes - on Tuesdays
* a las ocho y media - at eight thirty (8:30)
* de la mañana - in the morning
It's a question about someone's location or activity at a specific time on Tuesdays.