Tomorrow is the Hindu festival of Krishna Janmashtami - Lord Krishna's birthday. In Maharashtra, the state where Mumbai is, this is a huge celebration. Here is a description taken from Wikipedia about this festival. I'll be going along tomorrow to join in with the fun.
Janmaashtami, popularly known in Mumbai and Pune as Dahi Handi, is celebrated with enormous zeal and enthusiasm. The handi is a clay pot filled with buttermilk that is positioned at a convenient height prior to the event. The topmost person on the human pyramid tries to break the handi by hitting it with a blunt object. Mostly nariyal (coconut) is preferred being a sign of purity, truth etc. in Hinduism. When that happens the buttermilk is spilled over the entire group, symbolizing their achievement through unity. Handis are set up around the city, and groups of youngsters, called Govinda Pathaks, travel around in trucks trying to break as many handis as possible during the day.
Many such Govinda Pathaks compete with each other, especially for the handis that dole out hefty rewards. The event, in recent times, has gathered a political flavor, and it is common for political parties and rich community groups to offer prizes amounting to lakhs of rupees.