I keep my rabbit outside 24/7 all throughout the year. The key to keeping rabbits outside during the hotter months is to supply them with fresh water, proper ventilation, and something to help them cool down. Different people use different things but I personally take 2 liter bottles and freeze them, then plop those in the cage with my girl. She lays against it and licks it, so it keeps her nice and cool. This may be an additional thing but when refilling her bottle, I wet my hands a little - make them damp but not sopping wet - and rub the water on her ears. Rabbits control their temperature through their ears, so I like to make sure I can help her cool down a little.
It also doesn't hurt to put them somewhere nice and shady - direct sunlight can kill a rabbit. I placed Heidi's pen underneath our huge silver maple so she gets plenty of shade.
She also lounges in the cat carrier I put in her cage, so be sure to put something like that in there, too! It is also a good idea to serve their greens to them wet. They get water from the greens and then the water they're soaked in helps, too.
As for the cold, rabbits are typically good to go for temperatures as low as 20F, provided they have plenty of hay/bedding to burrow down in. I stuff Heidi's cage with it and the carrier she goes in, though she usually throws every bit of it out by morning. Rabbits do much better in cold than heat, so don't worry as much in the winter; we had an uncharacteristically cold winter this year and Heidi did fine in single-digit weather. I did feed her a little more than necessary during the time and a couple months before winter, hoping to fatten her up a little so she could deal with the winter better. Already, she's lost the weight since I started regulating her again - I know a lot of people might wag their fingers at me for overfeeding, but I think it helped. It certainly didn't hurt her. ^ ^ She's been outside for nearly 3 years now and she's perfectly fine; if given the right accommodations, rabbits are perfectly fine outside.
