It is similar to Unification Day; the official holiday celebrating of the end of the Unification War, which saw former Independent Planets like Persephone, unite with the Central Planets to form the Alliance.
As part of the festivities, Alliance-friendly "Persephonites" drape flags, bunting and pennants from balconies and between buildings. People can be seen in the streets wearing badges, capes, hats and much more emblazened with the flag of Alliance. Revellers frequent pubs, bars and taverns across Persephone; drinking, pontificating about the benefits of Alliance membership and their shared dislike of the Independents, and singing the anthem of the Alliance with pride.
Serving and retired soldiers proudly wear their military uniforms in public, some doning their medals; particularly those who fought against the Independents during the War.
Many with a deep dislike of the Independents will go to great lengths to show their patriotism. It is not uncommon to find the phrase, "DEATH TO ALL TRAITORS" graffitied on walls in the shadier parts of Eavesdown. 
- It is assumed that Alliance Day is celebrated on November 20th, the release date of Big Damn Hero. This would be similar to Unification Day, celebrated on the original air-date of Firefly (September 20th).