It is a sign between Me and the Children of Israel for ever; for in six days the G‑d made heaven and earth, and on the seventh day He ceased from work and rested (vayinofash). (ibid. 31:17) Why do we say at the beginning of Shabbat “vayinofash”? – as the Talmud says “Oy, how I lost my soul!”⁵ prayer of Shabbat Eve.**According to Chazal, this is to remind us of the additional soul that we receive on the Shabbat. They interpret the word vayinofash as “Vay, nefesh!” or, in their words: “Since it is Shabbat, Vay (Oy), how I lost my nefesh – my soul! (Beitzah 16a; Zohar 3:204b.) The proper time to say this should be motzoi Shabbos. The reason is to increase our love and joy of the holy Shabbat, by remembering that afterward, the great additional soul will leave us. Zichron Zos, hashmatos, parashas Ki Sisa⁶, Matos.

footnotes: ⁵ The above verse from Exodus 31 is recited in the Amidah ⁶ See also Divrei Emet