They have to be at least 8 weeks old, but I have never heard that there is a special time of the year in which to get one. They should stay indoors for at least two days, even longer, so they can impress on their surroundings and recognise their territory. Some friends of mine have never let their cats out, they stay in all the time as they live in a flat. The cats do not seem to suffer for this, indeed happier cats I have yet to meet. But there are two of them to keep each other company.
Please supervise it/them when he/she/they do go out. I would try putting your cat on a lead from it being little and let it get used to wearing a body harness rather than a collar. You can then train it to go for walks with you like a dog. Always get something recommended by a reputable pet shop though as if your cat is allowed out on its own it can get trapped or snagged if it is wearing the wrong thing. SOmetimes these harnesses are only for going out on a lead with, not wearing all the time. If you are traditional and let your cat go out all the time, then you need to de-flea it regularly, with "Frontline" from the vets or something similar. If you live near fields beware of ticks and get something to paint on them if your cat brings any back. Leaving them can make a cat quite ill.
Best to get plenty of advice from the vet or the RSPCA before you get one. And remember that the RSPCA (if there is one near you) always has cats and kittens to rehome (unless you are looking for a pure breed). If you go for a pure bred, it might be safer to keep it as an indoor cat as people will possibly try to steal it. That happened to a friend of ours who lost her cat that way.
Still, listen to me spouting off!! Hope it has helped and not put you off. I have had cats for 12 years now and loved every one of them.
Good luck