This is how I configure it on my server:
0 19 * * 1,3,5 root bash /home/divo/data/support_files/support_files_inc_backup.sh
The above command will run my script at 19:00 on Monday, Wednesday, and Friday.
NB: For cron entries for day of the week (dow)
0 = Sunday
1 = Monday
2 = Tuesday
3 = Wednesday
4 = Thursday
5 = Friday
6 = Saturday