"FMO" == F. Michael Orr <michael_or...@yahoo.com>:
FMO> 'at' the script to run the first time at the appropriate time on
FMO> the first Tuesday that you want. Have the script run, and if all goes
FMO> well, make the last line of the script be:
FMO>
FMO> at now + 21 days 'scriptnamehere'
FMO>
FMO> If it is a long running script, do that first. Either way, the script
FMO> will then be scheduled to run again in three weeks.

And if for some reason the script fails to run once, it will never run
again... Have you heard of graceful failure?