Example of what you could do, assuming a 30 day month:
Days 1-6 reserve
Day 7 off
Days 8-13 reserve
Day 14 off
Days 15-20 reserve
Days 21-29 off
Day 30 reserve (hopefully you’ll be on reserve days 1-5 of the following month so you don’t have to have a single day on reserve)
The above is legal but not very likely.