When to restart your hot tub
So when? As soon as possible.
Of course this question implies that you did not enjoy your spa during the cold season and that it was working to the bare minimum in order to let filtration or heating run or that it was completely drained.
But the principle remains the same as for a swimming pool: you have to restart as soon as possible in the spring when the ice is melted and the sun is strong.
Why so early ?
Simply because heat is algae and microorganisms best friend. It is very easy to start your spa when the water is clear, before algae and microorganisms invade standing water. Ideally, start your spa when the ice melts. Be aware that once the engine is running, even if the outside temperature drops below freezing, there are no problems. Also just do it to lift your spirit.