A Mask for All Occasions is related to the seasonal world event Hallow's End. It was previously part of the criteria of , 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.
Flimsy Masks can be found within from Trick or Treating with innkeepers during Hallow's End. You have about a 30% chance to get a Treat versus a Trick (typically a costume), and a 25% chance that your will contain some mask. This equates to less than 1% chance per Trick or Treat to get each mask. You can Trick or Treat once per hour.
Even though the Hallow's End event lasts for 2 weeks, it is very difficult to complete this achievement in a single year.