On average, about 20 percent of graduates never find out about reunions because no one can find them. So only about 80% know.
The Princeton Reunions are an annual college reunion event held every year on the weekend before commencement at Princeton University. It is known as the best-attended college reunion in the world.]citation needed[ Known simply as "Reunions", this event brings back to campus upwards of 20,000 alumni, and at least that many guests, for a four-day celebration featuring large outdoor tents, elaborate costumes, sporting events, alumni and faculty presentations, fireworks, and bands from rock to swing.
A Princeton Companion  places the advent of Princeton reunions shortly after the end of the Civil War. The 1890s (especially the University's 150th anniversary in 1896) saw increasing interest, although it was not until the 1950s that Reunions took on today's level of organization, particularly with respect to on-campus housing for returning alums.
A class reunion is a meeting of former classmates, typically organized at or near their former school by one of the class on or around an anniversary of their graduation. Former teachers may be invited as well. They reminisce about their school days and bring each other up to date on what has happened to each of them since they went their separate ways.
Some class reunions also include a reunion class gift to the institution. Reunions
High School Reunion