In surviving documents from the Middle Ages, it is not uncommon to see references to a period of time consisting of a year and a day. In 12th and 13th century custumals from the Holy Roman Empire, for instance, it is fairly common that ministeriales who engage in actions prescribed as illegal by their lord have a year and one day to regain his grace. Does anyone know when and where this custom originated, and why it seems to have been fairly popular in the Middle Ages?

As you say, it turns up all over the place, marriage, murder, sentence length etc.. I've never seen an explanation as to origin, but maybe it was to avoid quibbles over leap years, or to establish that at least a year had passed, never mind what time of day or night the days started or finished (sort of, at least a whole year - if you get my drift).
Geoff

If it is desired that the period of time end on the same date that it started, one must add an extra day. As an example, a time period starting on January 1st would end on December 31st without the extra day.

In pagan issues, the lunar calendar is considered. Adding the extra day makes our standard solar calendar coincide with the lunar calendar. The lunar calendar has 13 months with 28 days in each, making a year total 364 days. Our standard solar calendar has 365 days, making the extra day significant.

Thanks Nathan and Geoff. Both of your answers are probably reasons for the tradition.