Call to Arms: Battlegrounds! - 3 Weekends a Month
Calls to Arms occur during most weekends, starting on Thursday night at midnight and continuing until Tuesday morning. During a Call to Arms, emmissaries from that Battleground will be found in the major cities, and honor/faction rewards for performing objectives in that battleground are increased. Check out the Events Calendar for exact dates.

Elemental Invasions - Every Few Days
Reports of elemental incursions in different parts of Kalimdor are increasing. Every few days, a new wave of elementals blasts its way into the regions of Silithus, Un'Goro Crater, Azshara, and Winterspring - ostensibly, for the sole reason of seeing just how far into these territories they can penetrate before being beaten back by the forces of the Horde or the Alliance. Investigate these regions and aid your allies in countering these mysterious invasions.