Saints are special dates which are usually defined in Catholic countries. Every day of the year is assigned the name of a Saint. For example July 12th is always "Saint Olivier". Usually, each list of Saints does not change from one year to the next, but if required you can specify that one set be used until a given year and another set be used from that year onward.
Namedays are mainly defined in Scandinavian countries. Namedays are similar to saints but usually carry no religious significance. In some Scandinavian countries, you must use one of the available namedays of the calendar to name your child, the list of namedays may change from year to year, and there can be one, two or as many as eight names in a given day, while in other countries each diary producer is free to make up its own list of namedays.