Angelab,
In the past I have found Tuesday to be a good day to arrive in SJPdP. Those who have traveled during the weekend to start on Monday will have left and town is slightly calmer.
Here is a handy accommodation list from the local tourist bureau
http://www.st-jean-pied-de-port.fr/tourisme/hebergement/
When you arrive in SJPdP do stop at the Pilgrim Office located on the camino at 39 rue de La Citadelle.
The office is open every day and in season from early in the morning until late at night ie after the arrival of the last bus/train. Helpful multi-lingual volunteers can provide you with a Credential if needed, as well as a list of major albergues and current weather/ trail conditions. They can also help locate a bed/bunk for the night. Most importantly the office is a great spot to meet/ greet other pilgrims. ...Do plan to stop and chat
Bon chemin and Buen camino!.