When Seder night falls on Friday night, we recite "Mizmor Le’David" and welcome the Shabbat with the recitation of "Lecha Dodi" just as we do on an ordinary Friday night. There are those who maintain that these should not be recited when Friday night is Yom Tob, arguing that it would be disrespectful to the Yom Tob to welcome Shabbat with a special greeting without also greeting Yom Tob. This is akin to extending a greeting to only one guest when two guests arrive. Hacham Ovadia Yosef, however, contends that we extend a "greeting" to Yom Tob when we announce Rosh Hodesh on the Shabbat preceding that month. On the Shabbat before Nissan, for example, when we announce the onset of the new month, we in effect "invite" the Yom Tob of Pesah which falls during that month. Thus, we do not need to be concerned about extending a specific greeting to Shabbat when the Seder night falls on Friday night.
After the Amida, we recite "Vayechulu" just as we do on an ordinary Friday night.
There is considerable controversy, however, as to whether in such a case we recite the "Me’en Sheba" blessing ("Magen Abot") which is customarily recited on Friday night. The recitation of this Beracha was instituted for the benefit of those who would arrive late to the Friday night prayer service during ancient times, when the Friday night service would take place outside in the fields. Those who arrived late would still be praying the Amida when the service concluded, and would thus have to walk home by themselves, which was dangerous. The Sages therefore enacted the recitation of "Me’en Sheba" to extend the service by several minutes, thereby ensuring that the latecomers would finish their prayer with the congregation and be able to return home along with the other congregants. Many Halachic authorities maintained that as the night of the Seder is a "Lel Shemurim" – a night when we are granted special protection – there is no reason to recite "Me’en Sheba." Just as the Ashkenazim have the practice of opening the doors to their homes toward the end of the Seder to express the fact that they have no fear of the Mazikin (harmful spirits) that roam outside, similarly, there was no concern on Pesah night for the safety of the latecomers walking home by themselves. And thus the Shulhan Aruch (Orah Haim 487; listen to audio recording for precise citation), based on Rav Nissim Gaon, writes that "Me’en Sheba" is not recited when the night of the Seder falls on Friday night. This was also the view of the Hid"a (Rav Haim Yosef David Azulai, 1724-1806), who noted that this was the position of the Arizal (Rav Yishak Luria of Safed, 1534-1572). Another prominent Halachic authority who accepted this opinion was Hacham Abraham Antebe (1765-1858) of Aleppo. Indeed, this is the general consensus among the Halachic authorities, and this is the position accepted by Hacham Ovadia Yosef, in several places in his writings.
However, the Rashash (Rav Shalom Sherabi, Yemen-Jerusalem, 1720-1777) maintained that "Me’en Sheba" is recited every Friday night, even if it is the night of the Seder. Hacham Ovadia Hedaya (1889-1969), in his Yaskil Abdi, writes that since many of the Rashash’s teachings originate from the teachings of Eliyahu Ha’nabi, they cannot be easily dismissed. Indeed, in consideration of the Rashash’s ruling, Hacham Ezra Attieh (Syria-Israel, 1887-1970) had the practice when the night of the Seder fell on Friday night to pray with the Ashkenazim, to ensure that he would not be asked to rule on the matter of "Me’en Sheba." Interestingly enough, Hacham Bension Abba Shaul (Israel, 1923-1998), who was a student of Hacham Ezra Attieh, would always pray Arbit on the night of the Seder in a Minyan of Ashkenazim, so that when this occurred on Friday night he would not have to rule on this matter. (This was told by his son, in the recently-published Haggada with the teachings of Hacham Bension.) These Rabbis were reluctant to issue a definitive ruling in light of the different opinions that exist, and Hacham Bension concluded that each community should follow its established custom. It is clear that the custom among Syrian Jews is not to recite "Me’en Sheba" when the Seder night falls on Shabbat, as documented in the book Derech Eretz, and thus this is the practice that our community should follow.
Summary: When the night of the Seder falls on Shabbat, we recite "Mizmor Le’David," "Lecha Dodi" and "Vayechulu" just as on an ordinary Friday night. Many communities – including the Syrian Jewish community – omit "Magen Abot" in such a case.
