Society pages in old newspapers would report on local gatherings, often with names of the family patriarch and out-of-town or well-known attendees. You can download a few examples from GenealogyBank’s free downloads page.
Search for family surnames and the words family reunion. Try adding a place if you get a lot of hits. Kemp also suggests searching for reunions of high schools and colleges and military units.
A subscription to GenealogyBank costs $19.95 per month or $89.95 per year. Many public libraries offer cardholders free access to its sister database, NewsBank, through their Web sites.
Also see the newspaper research guide in the February 2007 Family Tree Magazine.