To summon the demon, you need to do in order these:
1st Go to your room read the guide in the PC (wwit hthe little skull symbol)
2nd Go to the bathroom and in the mirror chose the demonic language option (forth option all the way down)
3rd Go to Master Zhen temple until in front of the Shrine where you fight spirits, you don't need to go inside it, just in front of it
4th Finally head back home and into your room you'll find a red candle by the window. Click on it to turn it on and then enjoy whatever choice you pick
The possibility to summon the demons on remains in effect during the day of Halloween, the moment you go off to sleep it stops, so you'll have to wait a full year to re do it again