Información del albergue de peregrinos Albergue de peregrinos del Monasterio de Zenarruza, en Ziortza: Precio, plazas, características, etc...
I can't see if it is possile to reserve but you could call them and ask if you speqk some Spanish?
I will look a little more to see if I find something.
Normally though, it is not possible to reserve at Donativos.
You will learn as you go
If you are in reasonable shape and walk from morning until 12-14-15 in the afternoon, most often you will have no problem getting a bed.
If you arrive at your end destination at 18-20-... then most probalby all the beds at the albergues will be taken (depending on the season you walk).
One thing to learn is that pilgrims have been walking on the Caminos for 1000 years...
Pilgrims are in the local Spanish songs, festivals and much more.
I have experienced myself that a woman drove 2 pilgrims to another town, no charge ofcourse, to help them as there were no more free beds..
The local people are friendly and understand the life of the pilgrims.
Don't worry to much.
At worse you have to take a taxi to another town and it may cost you some Euros, but I bet it will not break your budget.
Else,... if you are on a budget and you don'ty have a bed for the night, go to the Firestation/firemen, "Bomberos" in Spanish.
The firemen will set you up so you can sleep on the floor at the firestation.
It is an old Spanish tradition. (Policemen will also help if you approache them).
Flow with it.
I hope this helps.