Any time of year is ideal to have your roof installed as long as it is not too rainy, frosty, icy, windy, hot, or, or below 20 degrees (dangerously cold only for the workers and not the materials). A true professional is used to working in all of the above conditions and can adapt his/her practices to accommodate for the challenge that the weather may present.
Always install shingles in the summer the shingles have a small amount of tar on The backside that makes them stick together the heat from the sun melts the tar
The best time to install your roofing is when it's summer because when it's winter one won't feel like coming out of there homes and more safe.