The following entry will run command at midnight on the first and fifteenth day of each month as well as every Monday:
0 0 1,15 * 1 command
The following entry will run command every five minutes:
0,5,10,15,20,25,30,35,40,45,50,55 * * * * command
The following entry will run command at midnight during every day of february:
0 0 * 2 * command