Filled Festive Mug usually makes an appearance during New Years and the last day of the Midsummer Fire Festival. Upon drinking one you gain a 30 second slow fall buff.
Source[]
Filled Festive Mug is obtained during the celebration by using a Festive Keg when you have an 
[Empty Festive Mug].
Notes[]
Although the Filled Festive Mug has a duration of 24 hours, mugs obtained during the 2009/2010 New Year's event did not vanish when their duration ran out. As of this edit, they continue to persist, giving free unlimited slowfall (at least until its internal random amount of charges run out) to anyone lucky enough to have obtained one. They are also not soulbound, meaning they can be traded to other people (though not auctioned). Presumably this oversight will be corrected at some point, so enjoy it while it lasts!