Leil Haseder that falls out on Shabbat we do not say Birkat Me’ein Sheva. However, if one starts it, he should finish it and then say Baruch Shem Kvod Malchuto L’Olam Va’ed.
Explanation:
Shabbat 24b: Rava said, “Yom tov which falls on Shabbat the Chazzan goes to the Amud (Rashi- He says the bracha of Me’ein Sheva and Magen Avot) but he doesn’t need to mention Yom Tov because it is only because of Shabbat we say these brachot …even on Shabbat he does need to say it just to protect from Mazikin (Rashi since the Bet Knesset used to be outside the city in the fields and the rest of the week no one would go to Bet Knesset, rather they would pray in there house after work but on Shabbat everyone want to the Bet Knesset and they were afraid for those who would come late added extra Tefillot for the Shliach Tzibbur so everyone would leave together.)
The Sfat Emet asks why don’t we say on the night of Yom Tov, people also pray in the Bet Knesset?
He answers on Shabbat the Tzibbur would accept Shabbat early there were some who came late and we are worried for them. On Yom tov the Tzibbur would all come later to Tefilla together so there was less worry.
Rabbeinu Perachia wrote that since the nation is busy with Simchat Yom Tov not all came to e the Bet Knesset and they didn’t make them say Me’ein Sheva
The (Meyuchas) Ran writes there are Mazikin only Leil Shabbat and Wednesday but no other nights. So only on Shabbat they decreed to say Me’ein Sheva but Yom Tov which falls during the week there are no Mazikin. (What if it falls on Leil Revii?)
Yom Tov which falls on Shabbat we say Me’ein Sheva, we just don’t mention Yom Tov.
So rules the Shulchan Aruch 268:8 “The Shliach Tzibbur says Me’ein Sheva…Yom Tov which falls on Shabbat we don’t mention Yom Tov in the bracha.
By Leil Haseder (Shulchan Aruch 487:1) On Shabbat we don’t say Mein Sheva. The Mishna Berura explains it wasn’t decreed for Mazikin) since it is Leil Shimurim-from Mazikin.
Even though the Rashash said to say it on Leil Pesach and so too the Yaskil Avdi agrees, but most argue.
If one started the bracha:
Shut Teshurat Shai and Daat Torah say to finish the bracha until the end.
R’ Sharya Davlitzky if you remember after you say the name of Hashem finish Asher Kidishanu Likro et Hahallel and start hallel. If you remember afterwards should finish the bracha without Shem or Malchut and so ruled R’ Scheinberg so you should not say a Bracha levatala.
R’ Ovadia Yosef wrote that on Leil Pesach the Shliach Tzibbur should not say Mein Sheva…if he made a mistake and started the bracha he should not stop in the middle but finish it (so it should not be levatala) and finish Mekadesh HaShabbat and then say Baruch Shem immediately after. But no one should answer Amen.
