Do you have shell access on the server?
If so, check out man cron(5) (which you can read <a href="http://www.die.net/doc/linux/man/man5/crontab.5.html">HERE</a>).
A crontab will have one command per line. It'll start with five "time" fields: minute, hour, day-of-month, month, day-of-week. So you could set something to run at 2:00am every monday and thursday with this:
0 2 * * 1,4 /command/line/thing
You write that file, then run the crontab on that file, and it installs it to your crontab, and then on the 1st and 4th days of the week, at 2 hours and 0 minutes, it runs whatever command is specified. Ta da!
But you've got to have shell access to use it, or have a tool your host gives you.