If you want to tip, you can tip on just the days that housekeeping will service your room (and your checkout day if desired). The person who cleans your room on the last day is not necessarily the same person who cleans it during your stay.
Since you're staying 9 days/8 nights, you'll get full cleaning service on day 4 (arrival date counts as day 1 even if you arrive late that day). Then on day 8 you'll get trash & towel service.
You might want to put your tip in an envelope, write "For Housekeeping" (or something similar) on the front, and leave it on a bed side table not near any wastebasket.
In case you haven't rented
DVC points before, here are the details of the housekeeping services.
Trash & Towel Service:
Empty trash and replace liners in trash bins (trash removal done daily now)
Provide fresh bathroom linens
Replace bath amenities as needed
Replenish facial tissues, paper towels and toilet paper
Replenish coffee, sugar, cream and sweeteners
Full-Cleaning Service:
Includes everything included with Trash & Towel Service, plus the following:
Change the bed linens
Vacuum and dust the vacation villa
Clean the bathroom(s)
Clean the kitchen/kitchenette and wash the dishes