OH, perfect days are judged by having full health at the end of the WAVE rather than the end of the DAY. You opened my eyes Cymes. I'll have to be more careful next time before I spread my disinformation.
@Cymes regardless of what the wiki said, I have tried getting perfect days using a hammer and it never awarded them even though I had full health at the end of the day. Try it yourself and see if you get the same result -
@akira it doesn't award perfect days if you use a repair hammer. You can still get a perfect with Repairman, and if you've got a couple weapons with Leech mods perfect days become routine...