Fans widely assume that U-day is held in March; however, this is based on an alliance officer's statement (in Bushwhacked) that some say the Independents' cause ended in that valley. This can also be taken to mean that the battle was simply decisive. By comparison, the decisive battles of World War II were Kursk (summer 1943) and the Battle of the Bulge (ended January 1945), but Hitler didn't kill himself until April 1945, and V-E day is in May.
On the other hand, the turning point of the American Civil War (the inspiration for the Unification War) was at Gettysburg in 1863. The Siege of Petersburg was in March 1865; the series of battles lasted for nine months of brutual trench warfare and ended with the Confederate army being destroyed entirely (more people died in that group of battles than in the entire Vietnam, Iraq and Afghanistan wars combined). The war ended almost immediately thereafter. Similarly, the Pacific front of World War II ended within days of the dropping of the atomic bombs on Japan.
So based on historical analogs, it's possible that the war dragged on for months following Serenity Valley. Negotiations following the battle took a week, but might have either covered a general end to hostilities or been as simple as negotiations dealing with the Independents' retreat from Hera. We simply don't know when Unification Day is, and that's as Joss intended.
Personally, I prefer the idea of U-Day being the day Firefly was cancelled. 24.99.61.188 12:41, June 14, 2012 (UTC)