There are four public holidays in Spain in December and the first is one of the few not related to religion.
El Día de la Constitucion (or Constitution Day) recognises the moment when the country moved towards modernity. On the 6th December 1978, a new constitution was approved in Spain, thanks to a referendum. After Franco died in 1975, a new political system was required, there was a general election in June 1977 and the new government began to draw up a new constitution. 88% of voters backed the change, which shepherded in democracy, after years of being ruled by a dictator.
It is now a national holiday, which means that most businesses and schools will be closed and transport services may run reduced services. This year it falls on a Tuesday, which means that many businesses will also observe a ‘puente’ and not work on the Monday either.
There is another national holiday during the same week, La Fiesta de la Inmaculada Concepcíon (the Feast of the Immaculate Conception) on December 8th. This is a holy day of obligation and many Catholics will attend special church services. It falls this year on a Thursday so, be aware, that if you want to get anything done that week, Wednesday 7th is your best bet!