Scheduling a regular job in Linux

To execute the shell script myjob.sh every Friday and Saturday of July at 0,20 and 40 minutes past 10pm, do

crontab -e 

In the editor that opens up, enter

0,20,40 22-23 * 7 fri-sat /absolute/path/to/myjob.sh 

and save. You are done!
The format is for specifying jobs is

Min Hour DoM MoY DoW Cmd.

See Ian's article for more.