That depends on the severity of the bad day. What went wrong?
If it was just mildly upsetting, something important didn't go as planned, bad grades, whatever.. I'll just take a long, hot shower or bath and be away from people for awhile. Alone time to think and cope and relax. Alternatively, old Retsupurae videos cheer me up sometimes.
If it was another night filled with nightmares and a whole 2 hours of sleep in intervals, I'll curl up with a cute, mind-numbing, tedious game to clear my head and then take a nap. Animal Crossing errand-running and Pokemon breeding are both great for that.
Ooorrr if someone hit some kind of trigger and my mood just got completely ♥♥♥♥ on for the next several hours, I will curl up on my couch with a shooter and a box (or 10) of chocolatey snacks. If someone else is around when this happens, I don't have the heart to take it out on them so we'll just watch a cute show together that will lift my mood. Leverage has been my go-to lately. (Parker is just so darned cute!)