I do not think it is a good idea for my daughter to skip a day of piano to have a sleepover.
What is your reason for not allowing her to have fun for a day.
When I'm a kid, my parents punished me for it.
It is wrong to punish a child when they should be having fun. It is ok to let kids be kids sometimes.
I wasn't able to have fun as a kid which means I am not going to let her skip her piano lesson for a sleepover. She is very much like me.
If you do not allow your kid to break from routine every now and then they may end up resenting things like the piano. There should be some sort of medium to practicing the piano so your child can stay sane.