Just as every farm operates with an independent set of circumstances, and every farmer is unique themselves, calving season is going to look different on every operation. There is no one right method or time of year to calve a cow herd.

Relevant links: 

Calving Season Timing and Transition (article)

Calving and Calf Management (webpage)

Calf 911 Resources

Records for Tracking Genetic Improvements (e-course)