Hello,
A Holy Year is a year when a saint's feast day falls on a Sunday. The feast day for Santiago (St James) is July 25. So, in 2010, this falls on a Sunday. I think that the last Holy Year for Santiago was in 2004. The reason why it is so popular, as far as I understand, is that pilgrims arriving in Santiago in a Holy Year can have their sins forgiven due to some long standing arrangement with the Vatican (I guess you would call it an Indulgence). You don't have to arrive on the feast day, the entire year is good for this. There is a special door on the cathedral that is only opened during a Holy Year. Arriving in Santiago during a Holy Year is a very popular thing for Spanish people to do. I met many Spaniards on the Frances last year who were only going part of the way, doing a bit more this year and then finishing off in 2010.
Other saints, of course, have their own Holy Years. For example, this year on the Portuguese route, I was in Ponte de Lima where they were having celebrations for the end of the Holy Year for the Virgin Peregrina (which means that her feast day was on a Sunday last year).
The number of pilgrims in a Holy year does significantly increase. For example, last night I was looking at the guide book for the Camino Ingles (starting from La Coruna or Ferrol). The numbers of pilgrims arriving in Santiago via that route per year tells the story: In 2003, 260 did this route, in 2004 (a Holy Year) 3,096 people did it. In 2005, it dropped back to 651. Like all the other routes, this one is gradually gaining popularity. So, in 2006, 840 people did it and in 2007 this number increased to 1, 085.
What does this mean for the Northern Route? Well, I expect that you will see many more people on it than in a normal year. You may well find people purposefully taking this route to avoid what everyone expects will be over crowding on the Frances. The people in charge of the various caminos have worked hard on setting up albergues and improving way-marking on all of the routes in the hopes that some of the pressure will be taken off the Frances. I would expect that the closer you get to Santiago, the larger the crowds will be. There will be the many people who are planning to finish in 2010. There will also be a lot of people doing the last 100km of the various routes in order to get a compostella.