The Seder begins with kiddush, which expresses the sanctity of the Jewish people and of the Pesaḥ holiday. The kiddush of Shabbat and other holidays contains the phrase “in commemoration of the Exodus from Egypt,” for the source of Israel’s sanctity began to reveal itself with the Exodus from Egypt, when it was made known that God chose Israel to be His special nation. At the Seder, on the night we left Egypt and are commanded to tell the Exodus story, the importance of kiddush is thus compounded. It is therefore fitting to begin the Seder with it.
Thus, unlike other kiddushim where only one person recites kiddush and drinks the majority of a cup of wine, on the Seder night each participant is poured a cup of wine, and after kiddush everyone reclines and drinks most of the wine in his cup. This is the first of the four cups of wine.
The rabbinic enactment to recite kiddush over wine expresses an important principle in Judaism. People tend to think that sanctity manifests itself in the spiritual realm alone, through prayer and Torah study, assuming that the more one denies the body, the more sanctity he attains. Yet, the fact that the Sages instituted kiddush over wine teaches us that sanctity can infuse and find expression even through physical food. This is true not only of the staple foods necessary for human sustenance, but even of wine, which brings people joy. Israel’s sanctity can be revealed in its totality specifically through the fullness of life, which combines the truth of Torah and faith with happiness and joy. We therefore recite kiddush over wine.
As on every Yom Tov, we recite the berakha of “she-heḥeyanu,” blessing God “Who has given us life, sustained us, and guided us to reach this season,” because Yom Tov is a mitzva that is celebrated anew during a specific season. The Sages inserted she-heḥeyanu at the end of kiddush; after declaring the day’s sanctity, it is only fitting to bless and thank God for having guided us to this sacred time. If one neglects to recite the she-heḥeyanu after kiddush, he must recite it whenever he remembers, as long as Pesaḥ has not ended.
Many people say a preliminary statement of intention (“hineni mukhan” or “le-shem yiḥud”) before each of the four cups of wine. One should not do so between the berakha and drinking, as this constitutes an interruption. Rather, the formula should be recited before kiddush, and in the case of the other three cups, before the berakha on the wine (MB 473:1).
If the first day of Pesaḥ coincides with Shabbat, we invoke Shabbat in the kiddush. If it begins on Saturday night, two berakhot are added: on the creation of fire (“borei me’orei ha-esh”) and on the separation of different forms of sanctity (“Ha-mavdil bein kodesh le-kodesh”) (SA 473:2).