crontab -e

0 3 * * 0 reboot

The first number denotes the minute (0-59).  In this case, 0 represents the top of the hour.

The second number denotes the hour (0-23).  In this case, 3 represents 3 AM.

The third number denotes the day of the month (1-31).  In this case, * matches any day of the month, so this field is ignored.

The fourth number denotes the month of the year (1-12).  In this case, * matches any day of the month, so this field is ignored.

The fifth number denotes the day of the week (0-6) beginning with Sunday.  In this case, 0 represents Sunday.

The final item is the command to be run.  The crontab runs with root privileges, so it has power to reboot the device.  A full path, as well as arguments, can be entered here if desired.