Rez Day parties are events based on an AV's "Born" date as seen in their Profile (which is based on when the user created the avatar), and are analogous to a birthday in real life. The scope and nature of the celebrations vary greatly, just as they do in real life. The term Rez Day came into vogue to distinguish between SL and real life birthday parties.
Generally, once a year, the avatar celebrates their rez day with friends and family at, appropriately, a "rez day party". Usually very similar to birthday parties, the avatar can receive gifts and "cards" (usually notes, but sometimes card-shaped objects with textures, scripts, and other decorations).