It is a scavenger hunt on bikes.
You are given a list of 8 road or place names, you need to get to them all and then to the finish point as quickly as possible.
They are themed each month and this one is the halloween-cat, so people will come dressed as pretty much anything.
You can go alone or in teams of up to 3.
This is just the Oxford ones though, they will be different in different cities.
Do you know Oxford?