In general, as on all festivals, we wear our finest clothing. One should try to the best of one's ability to buy new clothing for one's family in honor of this great day, and wear nice clothing in honor of the entire holiday which lasts seven days (or eight days outside of Israel). In fact, a husband is required to buy his wife a gift of clothing, jewelry or something else that makes her happy in honor of the festival.
Regarding wearing white, it is customary among Ashkenazim for the head of the household to wear a plain white garment or robe, called a kittel in Yiddish, during the Seder. Several reasons are given for this custom:
Curiously enough, in some ways the wearing of white on the Seder night is associated with death, as the dead are buried in white burial shrouds. One reason for this is to instill a feeling of humility as we recline, to recall our humble beginnings and appreciate God's mercy in having taken us out of slavery. The hard boiled eggs eaten at the Seder (in addition to representing the festival-offering) are considered a mourner's food and also conjure up this idea of humility. Of course, the lowly, unleavened matzah clearly communicates this night's emphasis on humility. Another reason for this curious undercurrent of death and mourning at the festive meal is that Tisha b'Av, the day of the destruction of the Temples, always occurs on the same day of the week as the first night of Passover.
However, other commentators interpret the custom of wearing white quite differently. They maintain that there is nothing finer than to officiate the Seder in a plain white garment. For it was thus that the High Priest entered the Holy of Holies, the innermost sanctuary, once a year at the pinnacle of the Yom Kippur service. On this night, each Jew who celebrates the sacred Seder is like the High Priest performing the most holy service.
This custom of wearing a white garment at the Seder is not observed among Sephardi Jews. To this day, many Yemenite Jews wear traditional garb at the Seder, including colorful robes and turbans. Some have the custom to reenact the Exodus in a small type of skit during the Seder.
