Stratford Mop festival, so called because it was traditionally a fair for hiring seasonal workers, who would stand around with a tool of their trade (a mop, for instance).
Any excuse for a bit of fun, there used to also be a few stalls there.
No longer needed for hiring, it's now just a funfair.
There's another, smaller, 'Runaway mop' in a month or so for those workers who didn't get along with their employers (and vice versa) to try again. Now just another, smaller funfair.
