I recommend advance reservations for the period from Holy Thursday (the Thursday before Easter Sunday) through the following Sunday (First Sunday after Easter). Typically, many Europeans have Holy Thursday, Good Friday and Easter Monday off work as a official holidays. Because of this, many folks can walk a segment of the Camino for up to 10 days without having to use much of their personal holiday or vacation days.
During this period, especially on the more popular Camino routes, including the Portuguese, there is a short-term surge or "pig in the python" effect. This wave of pilgrims places a short-term, but possibly inconvenient strain on accommodations along the way.
Also, Semana Santa is a REALLY BIG deal in both Spain and Portugal. So expect crowds in the cities and larger towns.
I hope this helps.