When to go
Although you can walk the Camino at any time of year, the 'pilgrimage season' starts around the Holy Week (the week before Easter) and lasts until mid-October. The most crowded month is usually August.
Weather is quite diverse on the Iberian peninsula. The Pyrenees are cool even in summer, and can get quite cold and snowy in winter. The interior regions have a mediterranean climate. And finally, Galicia is famous for its rains all year round.
If you're walking the Camino Portugués or the Camino del Norte, the climate there is mostly atlantic since the route is close to the ocean.
Summer is generally hot in Spain and Portugal, so if you prefer walking among cooler conditions, choose late spring or early autumn. These are also far less busy periods on the Camino.
Snow is not at all rare in Spain and northern Portugal in winter. Sometimes (although this is not a common event) snowfalls result in impassable roads, and can cause a delay of one or two days.
In summer, there are more people and generally a bigger 'rush for beds' (pilgrims rising early to avoid the midday heat and to arrive early at their next stop in order to secure a bed for themselves), although usually everyone finds themself a place to sleep. Still, you should be prepared for more albergues being already full when you arrive there.
Many people start their pilgrimage in late June and early July, timing their arrival in Santiago de Compostela to 25 July, when the city celebrates the feast of Saint James over several days.
The weekend prior to 12 October, the Spanish national holiday, is a fiesta in Spain and accommodation might be hard to find in the big cities.
Accommodations and facilities
Many albergues and refugios are open traditionally from the Holy Week until mid-October. In the cold months, there are less albergues you can find open, so plan carefully if you're doing an 'off-season' Camino.
In smaller villages some restaurants, bars and hotels may also be closed in winter.