Three out of four pilgrims choose to walk the Camino de Santiago during summer. The reasons are obvious; summer is the time of year when most people, including students, have time off. And besides, the good weather is almost guaranteed most of the season.
However, there is a big disadvantage to this: there will be a multitude of people walking the Camino. This could cause you to miss the spiritual atmosphere of the Camino making it more like a tourist route. In addition, you can have complications in finding a place to stay the night.
Summer is definitely not the best time to do the Camino, the reason is because during daytime, the temperatures can be too high, and more than one pilgrim has been forced to stop the walk due to a heatstroke. This is why you should be very careful and not walk during hot peak hours (between noon and 4 pm).
Winter is also not the best time to do the pilgrimage, especially on some routes such as the Primitive Road, the North or the Portuguese. Rain and snow can make walking practically impossible due to the mud and puddles that are formed on many of the roads.
The best time to do the Camino is in spring or early fall. During that time, the temperatures are pleasant but not extreme, and the paths are not so crowded. Finding accommodations will also be much easier, you will be able to better experience the Camino and its spiritual magic.