Vicksburg – Described by Lincoln as the key to winning the Civil War. Jefferson Davis, confederate president, described it as the nail that held the South's two halves together. Vicksburg sits on the Mississippi river.
The Vicar's burger (Vicksburg) contained a key (Lincoln's key) and a nail (Jefferson Davis's nail). The Vicar didn't eat the burger hinting at the fact that, after 42 days of being under siege with no food the confederates surrendered.
The Vicar tried to eat the burger the day after getting the burger (the day after Gettysburg).
The battle of Gettysburg was fought from July 1-3, 1863. The day after this the confederates at Vicksburg surrendered.