I want to be very careful about how I proceed here. By "day" I merely mean a chunk of time. It can be literally one day (it will be for me) or you can stretch it out over a couple of days. Anyway, I'm digressing already. So here's my strategy for the first day.
Lay down the law. As you saw in the intro, my kids are always hanging out in my room and dumping their stuff in there. I have now made it very, very clear that the space belongs to Mom and Dad. All others may only enter upon permission. Anything that goes into the room with a child comes out of the room with the child. Absolutely no Legos. Do you need a similar law? Do it. Do it with kindness and compassion, without blame, but be firm.
If this sounds a little harsh, remember: this is a haven and a sanctuary. Not a closet. Not a playroom. They have the rest of the house for that.
Take care of the sheets. Take a look at your sheets and quilts or blankets. Are they in good condition? Do you love them? If so, wash them, then make the bed. If your bed linens needs some help, try to get new if you can afford it. If you can't, wash and dry them and make your bed as best you can. Having bed clothes you love can actually go a long way.
Remove all obvious trash. Any trash cans or bags of trash that need to be emptied? Go do it. I'm talking about obvious trash, not to go looking for it. That comes later.