Rain can really damage your home if you are caught without a completed roof, so dont do one if you are in a rainy period. These are predictable to some extent. Florida and the South it rains spring and summer. Southwest it rains mostly in the spring. In the north you want your new roof on before the snows. California it doesnt rain so anytime. So schedule this to fit the rain patterns and dont let the roofer tell you otherwise. He works for you.