Of course, the decision of when to walk the Camion will be determined by your own flexibility and ability to making space in your schedule. Even if you can choose to go at any time, there are other factors you will want to weigh when choosing when to walk. The information I present here will be related to the Camino Frances. If you choose to walk a different route, you can search the web for the information which pertains to it. Now, here are some of the things you will want to consider:
Many sections will be covered with snow in the winter months, November, December and January. March can be very cold and April and May can be unpredictable. I walked in June and we had only three days of rain, but the Mesata was unseasonably hot. July and August can be sweltering. The heat can remain through September. and October can be warm as well.
No matter what season you choose, you need to be prepared with appropriate clothing, and some ideas of how you are going to physically handle the weather conditions. Rather than bringing a heavy jacket, it would be better to bring layers of clothing to allow you to adjust what you are wearing to the temperature. Always keep yourself hydrated, and pay special attention to this if it is hot.
The Oficina de Acogida de Peregrinos in Santiago keeps statistics on the number of Compostelas issued in each month. There has been a marked increase in people walking the Camino over the last ten years, particularly in the summer months. Not everyone who walks the Camino receives the certificate marking their completions of the Camino so there are more pilgrims walking than the statistics indicate, and you only need to walk the last 100 km or bike the last 200 km to receive it, so the end of the Camino is definitely busier than the beginning.
Even with that caveat, these statistics provide us with some help in determining the best time to walk, especially if you want to avoid the largest crowds and thus have less trouble finding a bed at night. Here is what the number of Compostelas issued by month in 2015 looked like:
From this chart it is easy to see the most crowded months on the Camino are July and August. These are also the hottest months and so may not be the best time to choose to walk. The spring and the fall might be better times in regards to crowding, but you have to take into account the weather can be less predictable. People do walk in winter, but not many, because they have to contend with snow over the mountains.
Cultural and Religious Festivals
During Holy Years traffic increases significantly on the Camino. Holy Years take place every 25 years, unless the Pope declares a special one, as was the case in 2016 when a Year of Mercy was declared. In Santiago de Compestella, it is considered a Holy Year if the feast day of St. James, July 25th, falls on a Sunday. The next scheduled Holy Year is in 2021.
There are also other events, like the festival of San Fermin festival (the running of the bulls) in Pampolona, which is held every year from July 6-14. The crowds in the streets can be huge and it can be difficult to find accommodations, but it can also be an experience of a lifetime. When I was walking we miss a similar festival in Sahagun by a day because we did not know it was taking place.
It is your choice if you want to participate in, or avoid, such events. Either way, it would be wise to do a little reach through a guide book or on the internet to find out what will be taking place when you are thinking of walking. This information may tweak when you choose to go.
My personal preference would be middle to late May and into June, or September into October. However, with the right clothing and mental attitude, you can walk the Camino quite safely from early spring and into the fall. Only those well acquainted with hiking though snow should attempt a winter pilgrimage. You should also note, many of the Albergues close in the fall months through winter.