I worked around that problem this way
I use a base of 12 tapes in my rotation. That allows me to have two weeks of Differential backups and three weeks of Full backups, and finally, one tape for a monthly long-term archive. They are labeled like this:
Monday Week 1 (Week 2)
Tuesday Week 1 (Week 2)
Wednesday Week 1 (Week 2)
Thursday Week 1 (Week 2)
Friday Week 1 (Week 2, 3)
Friday Monthly Archive
Once created, I change the label on the tape (both physically and in BE) to the exact date of the archive and send it offsite for long-term storage.
Then I get a new tape, initialize it into BE and make it my new Friday Monthly Archive tape.
This way it really doesn't matter which Friday is getting backed up or if a given month has five Fridays. You will always be sure that there will be 28 days (four Fridays) from the time any of the Friday backup jobs will run.
I hope I've explained myself well enough. If you have any questions, I'd be happy to help.