Laws of the Seder
1. Although in general, one may recite Kiddush and begin eating one's meal
before nightfall on Shabbos and Yom Tov, and thus add from "non-holy time"
to "holy time" (1), this is not allowed on Pesach.
[The rationale is] that the mitzvah of eating matzah must be fulfilled at
night, [at the time] when the Pesach sacrifice [was to be eaten], as
[Exodus 12:8 states]: "And they shall eat the meat on this
night." Similarly, the mitzvah of drinking the four cups of wine must be
fulfilled at night. Since the cup [of wine] over which Kiddush is recited
is one of the these four cups, the Kiddush should not be recited until it
is definitely "night" (that is, after the appearance of three medium-size
[Shortly after nightfall], one should put on the kittel and sit at one's
place for the Seder. It is a mitzvah to give children almonds, nuts, and
the like to play with, so that they will notice that something is different
about this night, and ask about it. [Hopefully,] this will motivate them to
ask other questions concerning the matzoh, the marror, and the reclining (2).
When a child is old enough to be educated (3), and he appreciates the
sanctity of Yom Tov, and understands the narrative of the exodus from
Egypt, he should also be given a cup [of grape juice, or wine] to drink
from (4). It is customary to pour an additional cup of wine, besides the
cup given to each of the Seder participants. This is called Elijah's cup. (5)
(1) This concept is called "Le'hosif Mechol al Hakodesh," which generally
means to "bring in" Shabbos or Yom Tov before nightfall; once one has
consciously accepted Shabbos or Yom Tov, all their laws and restrictions apply.
(2) It is a mitzvah to do something at the start of the seder that is not
done on other nights of the year, in order to stimulate the children's
curiosity and draw them into asking why this night is different from all
other nights - "Ma Nishtana Halaila HaZeh..." Asking the "Four Questions"
during the seder is not supposed to be a ritual formality; rather, there is
an obligation to do things that will stimulate actual curiosity and
questioning, because answers do not make a difference to people who don't
have questions (See Rambam, Yad, Chometz U'Matzah 7:3).
(3) That is, when he reaches the age of "chinuch," which varies according
to the individual child's level of awareness.
(4) It is a mitzvah to have the child perform the mitzvah of the four cups,
however it is not an obligation, because many authorities ruled that the
mitzvah of the four cups was not ordained for children.
(5) From the wording of the Kitzur Shulchon Oruch, it appears that Elijah's
cup should be poured at the beginning of the Seder. This is the custom in
many communities. In others, however, Elijah's cup is not poured until
after the meal is concluded.