It depends on which season you trek, in which spring and autumn are the best time to travel to Everest base camp. During Spring, which is from March to May, it is a lot warmer and humid in which day time it goes above 200c and at night it stays around 1-50c. The Autumn season which is from September to November, is temperate weather for walking but morning and night after sunset and before sunrise is cold which falls below 00c.