In the terminal:
$ crontab -e
The first time you run crontab the system will ask which editor you'd like to use.
Select nano.
Copy and paste the following cron jobs at the end of the document:
@reboot python /home/pi/Desktop/sensor-butler/wifi-checker.py &
0 0 * * * python /home/pi/Desktop/sensor-butler/wifi-checker.py
The 1st line will run the script in the background at startup, the 2nd will run the script at midnight everyday.
We are calling wifi-checker.py and not pir.py because the first will
activate the sensor only if the RPi is connected to the Internet.