A period of time, slightly longer than normal, one goes without shaving. Ususally associated with the holidays and comes to an end in early January. Can be intentional or unintentional. In the common case of beards - the shavecation usually upsets a wife or girlfriend, and is often incorrectly mistaken for laziness.
Barbara: "Sheesh, you look like you've been on a month-long shavecation"
Walter J: "Don't worry boss, the beard will go at the end of my shavecation. See you next week"