It is customary that the person leading the Seder wear a plain white garment, or kittel. Some maintain that the basis for this custom is that the kittel resembles a burial shroud and thus serves to remind one of the futility of vanity and pride.
These commentators maintain that it is for the same reason that many eat hard boiled eggs at the Seder, for eggs are the food served to mourners. The white garment also serves to remind us of the destruction of the Bet haMikdash on the Ninth of Av, a date that always falls on the same day of the week as the first day of Passover.
Others interpret this quite differently and maintain that the white garment serves to remind us of the white garments that the Kohen Gadol would wear when he entered the Holy of Holies while serving in the Bet haMikdash. On this night, every Jew who leads the Seder service is considered to be like the Kohen Gadol performing the Divine service. Among Sephardic communities, there is no custom of wearing a white garment at the Seder.