By default crontab is not install on the raspberry Pi.
Install crontab, and edit a cronjob for root :
sudo apt-get update
sudo apt-get install cron
sudo crontab -e
Create the job (for root) :
# -------------------------------------------
# arret de la machine de dimanche a vendredi
30 20 * * 0-5 /sbin/shutdown -h now
# arret de la machine les samedis
30 23 * * 6 /sbin/shutdown -h now
Now, your raspberry will poweroff (shutdown) at 20:30 from sunday to friday and at 23:30 on saturday