Backup schedule If you set the retention to 7 days, then the first backup set will delete at the end of the second set / week, leaving you with 7 days. There is no way thought to keep the first full as that would be a week earlier and be missing the 6 incremental backups that followed it.
In this scenario with 7 day retention:
Day 1: Full
Day 2 - 7: Incremental
Day 8: Full
Day 9-14: Incremental
Day 1 - 7: Deleted after Day 14 backup
Then the process repeats. On the next day a new Full will run, followed by 6 incrementals. In this case, you would only have 1 day where you only have a single full as a second one will be created on the next chain.
Can you go into a little more detail about your use case so I understand it better?
Thanks.