Yarn Bombing

Also known as Yarn 'Storming', it is a type of graffiti or street art that employs colourful displays of knitted or crocheted cloth rather than paint or chalk.
I actually happened to be sitting in the living room with my dad when the subject of yarn bombing came on The One Show.

'While yarn installations – called yarn bombs or yarnstorms – may last for years, they are considered non-permanent, and, unlike graffiti, can be easily removed if necessary. Nonetheless, the practice is still technically illegal, though it is not often prosecuted vigorously.^[1]

While other forms of graffiti may be expressive, decorative, territorial, socio-political commentary, advertising or vandalism, yarn bombing was initially almost exclusively about reclaiming and personalizing sterile or cold public places.^[2] It has since developed with groups graffiti knitting and crocheting worldwide, each with their own agendas and public graffiti knitting projects being run.'

^[Taken from Wikipedia]

 This itself is an event that a group of people organise together. They tend to strike at midnight whereby they cover a whole space in wool and yarn.