When will be a good time to put my rabbit outdoors again?

Hi, i have one boy rabbit, hes name is shakespeare and when i first got him he was really small so i thought i would put him in the shed in his hutch so he doesnt get cold and use straw to keep him warmer and now he is older like ive had him since september and he has got bigger but i still keep his hutch is in the shed to make sure he is warm, what month would be a good month to put him back outside? he has a heater in the shed that i turn on when it get really cold so i dont want him to be put outside and then he gets cold so what time of the year will be warm enough for him to go outside?