Having shut off many fridges and freezers for storage, I can tell you exactly what happens and why.
It will quickly become contaminated with mold
The interior of a freezer has some plastic "skin" to present a surface that is more aesthetic and easier-to-clean than galvanized steel. However, the freezer does not stop at the skin; behind the skin is still the interior of the freezer. And air circulates on both sides of that skin as well as the galvanized metal behind it... and condensation and frosting occur on those hidden surfaces as well. In short, water gets to lots of inaccessible places you can't see or clean.
Try an experiment. Take a Ziploc bag you formerly used to hold food, put about 10 drops of water in it, puff it up, and leave it alone for a week. "What are you thinking, Harper? That will turn into a disgusting, moldy cesspool!" Exactly.
And the same exact thing happens inside a fridge or freezer when you shut it off, because, that door seal is so good, and because there is frost or condensate in places you can't reach. That melts, humidity reaches 100% in there, and the mold goes to town.
And the worst part is, much of the mold activity is actually inaccessible - on the backside of the skin or the metal parts. You'd have to tear the appliance apart to access those.
The cure is to wedge the door open
But, you have to do it everytime.
As long as the door is pushed open 20mm or so, outside air will circulate, and keep the humidity controlled. That will stop mold growth, provided you have mold/humidity under control in that room of course.
Watch your energy use with an older fridge...
As THelper says, newer fridges are considerably more efficient, and can be worth the cost and sustainability burden of replacing the fridge, in the energy savings on the fridge. You can measure its energy usage by a simple energy monitor like a $20 "Kill-a-Watt", and compare notes with others with new fridges.
Consider also your duty cycle: if it's powered down 3 days out of 4, factor that into the equation.
That also varies by district - if you're in a place like California or France where so much is sustainable or nuclear, it may not be worth it. That's too complex a calculation to do here.
And recycle that freon! PLEASE!
Older fridges use R-22 Freon. You may recall the hubbub in the early 90s as they abolished R-12 Freon from automobiles. Those systems function in a very harsh environment and so have a lot more leakage, so were by far the most urgent (and also much harder on the ozone layer than R-22 if I recall). These freons devastated the ozone layer - they're a huge part of why we must wear sunblock today. But it's on the road to recovery.
By treaty, international production of R-22 was phased down to half by 2005, 1/10 by 2015, and ZERO as of January 1, 2020. In much of the world, refrigeration shops are required to use systems to vampire out old Freon and recover it. This is either to be annihilated (broken down chemically into other molecules such as table salt), or re-refined and re-sold to maintain older systems. At this point this is the only possible source for R-22.
So if you decide it's time for a new non-R-22 freezer, please call around to a refrigeration shop to do the proper vampire procedure. DON'T just go all "Office Space" on it or toss it in a dumpster, because the piping will get damaged and it will hissssssss into the atmosphere - the worst possible outcome in all respects.
