I need a cron job to run every second weekday for example.
If cron job last ran on a Thursday it should run on the next Monday.
The re-run on Wednesday and again on Friday again and on Tuesday and again on Thursday bring it back to the start.
Run it every weekday, and have the script set a flag in a file that says whether it did the work the on the previous run. If it did, it skips the work that time and inverts the flag.