The 6th and 8th December are both national holidays.
The 6th December is Constitution Day. Constitution Day (Día de la Constitución) marks the anniversary of the referendum held in Spain on December 6, 1978. In this referendum, a new constitution was approved for Spain. This was an important step in Spain’s transition to becoming a constitutional monarchy and democracy.
Franco was head of state in Spain from April 1, 1939, until November 20, 1975. After he died, Spain needed a new constitution and so political system and elections were held on June 15, 1977. After a new parliament was formed, a new constitution was drawn up and approved by 88 percent of the people of Spain in a referendum on December 6, 1978. So as a transition from a dictatorship to modern day Spain, it is a fairly important anniversary. It is generally celebrated with a quiet relaxing family day at home .
As there is another national holiday on the 8th December, people often take the few days off, known as a “Puente” or bridge. This year, the 6th falls on a Friday so you will find the whole weekend (apart from the pre-Christmas festivities) pretty quiet.
The 8th December is a Holyday Day of Obligation of the Immaculate Conception of Mary (La Inmaculada Concepción de la Virgen María). The reason it is 8th December is that Pope Pius IX issued a document stating the importance of the Immaculate Conception in the Catholic Church, on December 8, 1854.
Many people think Immaculate Conception is referring to the conception of Christ through the Holy Spirit, but it is not (that is actually celebrated on 25th March). The Feast of the Immaculate Conception refers to the belief that Jesus’ mother Mary was conceived without sin and that God chose her to be Jesus’s mother. Many Christians and Catholics attend special church services on this day and it is a national bank holiday in many countries.
Spain has the same amount of bank holidays each year (eight National ones and four days that are particular to each region so 12 in total). In addition, each toen will have 2 more local holiday dates.