Summers are very hot in the Grand Canyon. Winters do not get as cold as one might expect but the low angle of the sun in a deep canyon means a lot of shade. There is also a micro climate induced by the cold water which cools the air. The canyon walls create an inversion for the cooler, heavier air near the river. The inversion is around 10 feet in height. This can lead to competition in the summer for campsites that are near the river and in the inversion.
April through June are the dry months. The monsoon season starts in July, peaks in August and may extend into September. While it may seem strange to talk about monsoons in the desert, this is the northern extent of the summer monsoons of Mexico. Not only can there be rain in the monsoon season but it often is very intense and flash floods are common. The Navajo or Dine that live just to the east of the Canyon call this heavy intense rain "male rain". If you are in the Canyon during the monsoon season remember to take this into account when choosing camps, locating tent sites and hiking side canyons.