Welcome anewcomer. I am also a newcomer to calving outdoors as i grew up around dairy cows calving year-round indoors. I am sure the ideal time is different for different locations, for me in Northern Ohio I think late spring early summer is best, around late April/ early May. I want the least amount of stress on the cow (Mud, cold weather, snow, etc) and I want her to be well fed with the new flush of grass to meet the high nutritional needs of nursing her calf. There is nothing better than calving in lush grass in my opinion. Plus, I'm not pre-occupied with feeding hay. Good luck!