A slumber party is a get together also known as a sleepover. Kids get together and they play games or whatever and they stay the night.
The "on" is not needed.
A: What are you doing Friday?
B: I have a date Friday.
A: Some of us are getting together New Year's Eve. Want to come?
B: I can't make it then. What are you doing New Year's Day?
- For Teachers