yeah... usually once every quarter, if the company gives dividend at all in the first place. Gucci, for example, actually give dividend once a year (and boy do they give alot). Meanwhile, most don't give any at all (Microsoft didn't until last year), and the dividends are soo little anyway that i personally don't really care about. Mortage firms and electric companies usually yield the highest divident/price ratio.
anyways, back to your question, i also have a hard time finding divdend yield dates on finance.yahoo.com and moneycentral.msn.com, they will however, tell you when the NEXT date is.
I do know ameritrade.com can give you the previous yield dates.