In my dream I had last night it was London Zombie Day.
I think how it worked was as follows:
In the morning 10 people in different parts of London are marked as 'Zombies' (I forget how) and then for the rest of the day they have to try and 'infect' as many people as possible, who then keep trying to 'infect' other people and the aim is to get through the entire day without being 'infected'. It was like this massive city-wide game of tag with everyone playing.
Seriously how awesome would this be?