Katniss' wedding dress is just another costume she puts on to please the Capitol. She didn't choose it, and it's for an event she doesn't really want to attend. This is only underscored when her wedding is canceled because both the bride and groom are heading into the arena for the Quarter Quell and only one of them can survive.
Yet the dress itself doesn't stay just a wedding dress. Cinna works his magic on it to make it a far more potent symbol, which, like the mockingjay, encourages the revolutionaries to move forward. Intrigued? For more on the importance of costume and Katniss' wedding dress, check out "Symbolism: The Mockingjay."