A Mask for All Occasions

A Mask for All Occasions is related to the seasonal world event Hallow's End. It was previously part of the criteria of Hallowed Be Thy Name but was removed in patch 3.0.3. If you destroy masks you have received, the criteria stay completed. Note that masks are BoP and cannot be traded.

Source
All the masks can be purchased from just outside Stormwind City and  in the Ruins of Lordaeron for. Some of them can be found within Handful of Treats and the Loot-Filled Pumpkin.