When to put him outside?

[Mimbulus]

I have a 5 year old Spur thigh (Testudo graeca). In the summer, on warmer days we let him roam the garden (it's escape proof) but at night bring him in to his "table". On the colder days we keep him in but let him wander the house.

I plan to build an appropriate outdoor area for him so he could be outside at night too - but was wondering over what time of year is it "normal" for him to be out overnight?

Thanks