Preparations for the Seder
6. One should prepare one's seat [at the Seder table] with the finest cushions one can afford, before the commencement of the festival, arranging it in a manner in which one can lean on one's left side (1). Even a left-handed person should lean to the left (2).
Similarly, one should prepare the Seder plate before the commencement of the holiday, so that as soon as one returns from the synagogue, one can begin the Seder without delay (3).
(1) It is a mitzvah on the night of Pesach to act and feel as if you, yourself, were just freed from slavery in Egypt. The Sages established the mitzvah of leaning to the left during the seder, as an outward expression of this newfound freedom; royality and nobility in those days reclined while eating (Rambam, 'Yad', Chometz U'Matzah 7:7).
(2) Leaning to the right was considered a health risk because doing so might cause the food to enter the windpipe instead of the esophagus (Mishna Berura 472:10).
(3) It is a mitzvah for a father to communicate the story of the Exodus from Egypt, to his children, on the first night of Pesach. Furthermore, as we saw earlier (HY 118:2), there is an obligation to do things during the Seder that will stimulate the children's curiosity and prompt them to ask "why is this night different." Therefore, one should start the Seder without delay, after nightfall, so that the children will be as awake and aware as possible.