Holy Years at Santiago de Compostela
Whenever St James's day (25th July) falls on a Sunday, the cathedral declares a Holy or Jubilee Year. Holy Years fall every 6, 5, 6, and 11 years: the most recent ones were 1982, 1993, 1999, 2004 and 2010. The next Holy Years will be 2021, 2027 and 2032.
The Puerta Santa (Holy Door), which gives access to the Cathedral from the Plaza de la Quintana is opened on 31st December on the eve of each Holy Year, and walled up again a year later.
As in the past, pilgrims reaching Santiago during a Holy Year, and fulfilling the conditions for it, are granted a plenary indulgence; and they are still years when particularly large numbers of pilgrims arrive.
As you have said yourself it will be busier and the negatives probably outweigh the positives unless you are Catholic and it is significant to you. The Camino is a safe environment in any year compared to the cities most of us live in.