According to the SAR stats, the death list and WFA/WFR training, the peak "danger time" on the summits is in late July and early August. This doesn't equate to it being the most dangerous per say it's just normally when the most unprepared people hit the trails and get in trouble. And this curve is very gaussian in the variations I have seen. But each season holds its own dangers. Winter has snow, cold, avalanches, ice falls, winds, white out conditions, hypothermia, glasading accidents, ice axe injuries, etc. Spring has avalanches, ski accidents, ice fall, post holing dangers, drastic temperature differences with unprepared people, etc. Summer has overcrowded trails, slick rocks, lightning, disorienting fog, dehydration, heat stroke, and drastic temperture differences (snow or rime while 80 in the valleys), etc. Fall has mixed trail conditions, slick rocks, post hole dangers, hypothermia, unprepared people expecting rides down, shortening days, etc. Working up here, each month has its own challenges and rescue needs. I tend to agree that late summer and early fall are our peak rescue season but nailing down which month is the most dangerous is like asking people what color is most beautiful or which weapon is the most dangerous. And rarely are you going to get two people to ever agree on this topic based on their own personal experiences.
Staff Meteorologist/Night Observer, KMWN (Mt Washington Obs., NH)