Hi,
Rain stick is a way to "spawn" FMC's friend Keade, so you can have fun at night with her as well.
To make it possible, you'll have to first successfully pass the party event that occurs on first Friday every loop and at the end of the party convince Kaede to stay over instead of going away (you should be tipsy, have enough 'impression' points and high enough base stats to do that).
Cheers