1) Almost every pueblo on the Francese has a church. 2) Most (not all) will have an evening mass, at 7 or 8 pm, depending on the season. 3) The time may not be posted or may not be posted accurately;
4) hospitaleros should know the time (some don't!), 5) the bartender nearest the church will know, 6) older villagers might know. As
@David Tallan suggests, Gitlitz and Davidson will give you great background on each church, and you will be able to annoy clergy for years to come with your knowledge of obscure Spanish saints.
In the past 8-10 years the Spanish bishops along the Camino Francese realized that this was no longer your typical rural village scenario, and they focussed on seeing Camino Francese churches relatively well supplied with clergy and have even tried to get multilingual clergy available -- do
Not count on this on other Caminos (on the del Norte and Primitivo, it was difficult to find evening masses and on the Catalan, nigh-impossible outside major towns).
In the same way that there is no overall official Camino authority in Spain, there seems to be no overall coordination of church times. Misa.org is often out of date and, in any case, schedules are vulnerable to the priest having a flat tire.... My advice to
@cwalden is to not expect to plan this from overseas (or even two villages down!), and to chance his arm and ask around on arrival, and turn up. Villagers and townsfolk will be happy to see
@cwalden and son. After mass, the priest will give you a sello for your credenciales.