Originally from http://callalilymean.com/calla-lily-care-bulbs-bouquet/
They have deeply rooted historical and cultural significance. They are popularly used in weddings and other holidays and occasions that are romantic in nature.
Just like other flowers, they have symbolic meaning that is distinct. They are traditional symbols of marital bliss, true devotion and divinity.
They are more specifically used to mark the sixth wedding anniversary. They have however also been used in funeral as a show of sympathy and purification of the soul departed. Calla lilies can essentially be used in any occasion in which rebirths, transitions and new beginnings are celebrated.
More on meaning of flowers at http://www.aboutflowers.com/flower-a-plant-information-and-photos/meanings-of-flowers.html