That being said, the sooner you tell them the better. I feel like that conversation won't be a bad one as long as you don't frame it badly. I personally would tell my assistant first since you work with them the closest and then your HS director and then finally your principal (some will disagree with me on that order of course). However I also would have all those conversations in the same day so that it doesn't give the chance of the rumor mill to blow it out of proportion or to blindside admin.
Just whatever you do don't burn any of your bridges. You never know what the future holds and if you ever do need to return to teaching for whatever reason you do not want to have set fire to those only to return to nothing left.
As far as the days go, I have no idea