Z-Day (pronounced Zee or Zed Day) was the name given to the zombie infestation of London that took place in the film Shaun of the Dead.
The two most noteworthy groups of survivors were the one led by Shaun (Ed, Liz, Dianne, David, and Barbara) and the one led by Yvonne (Declan, Mark, Maggie, Yvonne's Mum, and Tom)