The answer is July or August of each year. Why? Simple. That's when the new group of doctors in training, called residents, start their training programs. Other professions would call them "rookies." These doctors in training have just graduated medical school, and are now starting their career in their chosen medical specialty. Why is July and August the worst time to be a patient? Because these new doctors don't have the experience necessary to know exactly what to do, and more importantly, to know what NOT to do. Yes, they rely on their senior residents, and in some cases the attending doctors. However, in the middle of the night, when they're on call and handling multiple patient problems they are less likely to ask a senior resident or an attending physician for help- since they do not want to be seen as being inadequate and incapable of handling patient's medical problems.
The interns, also known as first-year residents, are the ones to deal with much of the 'scut work' that the senior residents do not want to do. Many are overwhelmed and mistakes do happen more often during these months by the interns than at other times of the year. There is a significant learning curve that each resident goes through in order to gain enough experience to be confident in their decision making abilities. If you are a patient in a hospital during this time, make sure you have a family member be your advocate. Check and question everything from pills that you receive to the medicine in the IV bag. Ask lots of questions and find out what alternatives are available. Importantly, ask the "rookies" is they've discussed your care with the attending physician and whether he or she agrees with the plan.