We have a relatively small barn with 6 large stalls in it, a feed room, and a small insulated room (we call it the closet) used for medical and grooming supplies.
Because of the size of our herd (50+), the barn is primarily used for foaling, a horse who is ill or injured, farrier work/grooming, or the occasional show horse.
About a month to six weeks before our first foal is due to be born, we disinfect all of the stalls and buckets. Larry prefers using a pressure washer with either bleach water or nolvasan.
At the same time I inventory our Foaling Kit and make sure we have plenty of Wormer, Banamine, Selenium/E, Gentle Iodine, mineral oil (for an enema) etc........ I make sure that all of our supplies are back in the kit or on the shelf next to it, so we know where things are.......During foaling season, some things do get moved around in our little medical "closet room". I also make sure the enema applicators, and feeding syringes are clean, etc......
This is also a good time to track down a colostrum source, if you weren't able to collect and freeze any the season before. (The vet usually helps there.) Oh, and this is when we throw out the old stuff, including the old Milk Test Strips and distilled water from the previous year and buy new!
And finally, I inventory our foal blankets, plus the barn towels and sheets.....making sure they are laundered and put in their rubbermaid chests that we use for storage.
That's just what WE do........Hope that helps?