I want to do a backup every Friday night (no, this is not the whole backup routine, just part of it). Each Friday night's backup will not be overwritten until 4 weeks later. So, essentially, I have a four revolving backups: Week1, week2, week3, and week4.

Now, I need the week1 backup script to run every 4 weeks. But I also want week2's script to run every four weeks. I know that I can tell the crontab to execute something every X weeks/days/hours/whatever. However, how do I set it up so that each of these four scripts actually run on different weeks, how do I avoid all 4 scripts running on the same night, then dutifully waiting for weeks only to all run again?

Thanks.