As the name implies, this is a holiday for the public i.e. for everyone. In the UK, it's commonly called a Bank Holiday.
All of the Canary Islands have the same Public Holidays of which there are 12 although mainland Spain have some different dates. However, the dates in the Canaries don't differ from year to year apart from the Easter dates.
Should a Public Holiday fall on a Sunday, then Monday is usually classed as a 'free' day which means that shops and businesses, etc will be closed on Monday instead. Although you might find that some shops clos on the Tuesday instead of the Monday :)