Sources: El Correo Gallego and Spain public holidays. The Galicia Autonomous Region's Xunta announced its selection of regional holidays for 2008, which will see the return of Día de San José (May 19) as a regional public holiday in Galicia, as well as the more frequently occurring Día das Letras Galegas (May 17, a Saturday, in 2008).
Analysis: There are 2 "standby" regional public holidays in Galicia: Día das Letras Galegas (May 17) and Día de San José (May 19), to be used in that order of preference, if any of the national public holidays should fall on a Sunday. Both were chosen in 2008, due to the fact that 2 of the national public holidays occur on a Sunday in 2008, Día de Reyes (Epiphany on January 6) and Día de la Hispanidad (October 12).
Recall that Spanish regions, towns, villages and cities can each have additional public holidays of their own. There is a base of, usually 9, public holidays that are common to all regions of Spain, plus Epiphany and Maundy Thursday which each region can replace with one or two region-specific public holidays. Regions also have the option of adding a regional public holiday, without removing any of the common ones, or Epiphany and Maundy Thursday.
The main reason why the specific public holidays for each province and municipality change every year, such as is the case here, is that these are usually patron saint days, which always occur on a fixed date. Therefore, when these fixed dates fall on a weekend, another holiday or patron saint is chosen for that year, to avoid "wasting" a holiday.