I gave my Euphorbias its first round of cold rain when our first good rain in Fall came last Nov 2, then allowed one day to drain, then brought it in. Typically they do not like cold temps. Cold and wet, bad combination, but in our area we have been too dry for such a long time, so it was okay then.
If they have gotten their good share of rain during summer, I would just bring them in and protect them from the cold. Position to an area where it will get as much sunlight. My temperature threshold for them is not to expose them below 50F if possible. But do expect some leaf drop as it adjusts from being outside to going indoors.
Just editing to add, I like looking at the forecast of accuweather, their temperature forecast includes a real feel gauge, since oftentimes the temps are further influenced by the wind chill. Check what you got for your area.
