If you choose to spend the night in Sahagún do your best to explore the town, it is not the prettiest of places but there are a few gems hiding about that give you a glimpse of its illustrious past. At one time it rivaled Burgos and León in importance and grandeur.
For reasons unknown the town hall has drawn the camino path through town in a way that skips nearly every building worth seeing, and which bypasses the plaza mayor entirely. If you want to explore you will have to leave the marked path.
A very nice certificate is available from the Santuario de la Virgen Peregrina on your way out of Sahagún, a type of half-way Compostela. See the map for the location as it is not on the camino proper.
Market day is Saturday. Winter pilrims can celebrate San Tirso on the 28th of January. San Juan de Sahagún on the 12th and 13th of June. Fiesta de Peregrina on the 2nd of July, and San Lorenzo on the 9th and 10th of August.
Sahagún owes its name to the 4th century martyr San Facundo, who along with San Primitivo were buried nearby. Because of this it has seen more than its share of battles between Christians and Muslims. Although the Christians ultimately won out, it was not before the town changed hands no fewer than five times. Each turnover saw the destruction of churches and their reconstruction. Today only 4 of the original 9 churches survive.