In the Amida on Shabbat night we recite Va-yekhulu, the three verses at the end of account of creation (Bereishit 2:1-3) that introduce the idea of Shabbat. The Sages teach (Shabbat 119b) that one who recites Va-yekhulu in the Shabbat evening prayers becomes like a partner with God in creation. God purposely created the world incomplete so that we could become His partners in its repair. Our partnership begins with belief in the Creator, and from there we work to rectify the world and express His glory.
The Sages further teach that one who recites Va-yekhulu in the Shabbat evening prayers is escorted home by two angels, who place their hands on his head and say: “Your iniquity will be removed and your sin will be atoned.” The idea of Shabbat is linked to repentance (teshuva), as attested by their phonetic similarity. Indeed, on Shabbat we are reminded of our faith in the Creator, and from that conviction we return (“shavim”) to all the good aspirations in our souls. One who recites Va-yekhulu on Shabbat eve expresses the profound significance of Shabbat and earns the opportunity for true repentance and atonement for his transgressions.
In the synagogue, the congregation recites Va-yekhulu again after the Amida. Some say that the public recitation of Va-yekhulu serves as testimony to the creation of the world (see Peninei Halakha: Shabbat 4:3). We say Va-yekhulu a third time in kiddush; often, important prayers are recited three times (as in Ashrei and Kedusha).
The Sages instituted that the ĥazan recites Magen Avot or the Berakha Me’ein Sheva (a summary of the seven berakhot of the Friday night Amida) after Ma’ariv, to serve as a sort of Ĥazarat Ha-shatz (see Peninei Halakha: Shabbat 4:2 for further study). This berakha is recited by the ĥazan, so after the congregation sings its opening passage Magen Avot, the ĥazan must repeat it by himself (MB 268:22).