Sure there is Holi, Eid, Christmas, with their attendant gaiety. But the biggest of all are Ganpati and New Year, and that's chiefly because of the crowds and the fact that both are synonymous with images.
The big one of course is the Lalbaugcha Raja, the idol for which a flyover has been demolished, to be rebuilt higher in order not to obstruct its passage to the sea. Ganpati going in a procession of hundreds of thousands of devotees is an icon for Mumbai.
The other big festival, one that brings the hundreds of thousands out, is New Year, of the Gregorian calendar, and usually celebrated out of doors, preferably at the Gateway of India. These are festivals in the real sense - involving the whole of Mumbai and not just a particular community.