In January, the coldest month on top of Mount Everest, temperatures regularly reach minus 35 degrees Celsius, without factoring in the wind chill. The wind regularly blows around 175 mph during the winter months, resulting in wind chill temperatures as low as minus 148 degrees Fahrenheit.
During the spring climbing season, temperatures on top of Everest are much milder. The vast majority of people who climb Everest do so in April and May, when the minimum temperature is only around minus 10 degrees Fahrenheit. More importantly, the wind doesn't blow during these months, meaning that the ascent is relatively free of the bitter wind-chill factor.