Your questions are not native, and mentioning the place ( her hous e) and time ( tomorrow ) will inevitably confuse the point of your question. What is the purpose of your practicing the musical piece? is your best chance for the answer you seek.
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Mister MicawberWhat is the purpose of your practicing the musical piece?That sounds a bit "tie and lace-up shoes" to me. I'd just say something like: At what function will you be playing the music you are going to practice at Mary's house tomorrow?