Unification Day is a holiday celebrated in the fictional universe of the science-fiction television series Firefly. It marks the day in which the Alliance forces defeated the resistance ("Browncoats") in the Unification War.
The Train Job", the second episode of the series (although Fox originally aired it before the pilot episode "Serenity"), opens with Mal, Zoe, and Jayne in a bar during a Unification Day celebration. Malcolm's brown coat and his disinterest in celebrating Unification Day lead to a brawl.
Unification Day is a holiday mentioned in the backstory of the television series Firefly. According to Nathan Fillion (Mal), the holiday is on September 20th.
There's also a Facebook page.