Big news anniversary dates are also good, in that you know about them ahead of time, and thus plan a web campaign around ahead of time. This November is the 45th anniverasry of the Kennedy assassination, next year the 40th anniverasries of Woodstock and the moon landing, the 25th anniversary of Nixon's resignation, the 10th anniversary of Clinton's impeachment trial, etc. Pick what you think is the best event to capitalize on, and have a ball.