Because God gave it that name (Exodus 16:23-25), a noun which originated in the verb (Genesis 2:3) that resulted from God having ceased from creating. See also:
They're called Shabbat candles (Neirot Shabbat in Hebrew).
Shabbat
Shabbat
They're called Shabbat candles (in English); neirot Shabbat (in Hebrew); or Shabbes Licht (in Yiddish).
Shabbat
The Sabbath day of rest is called shabbat in Hebrew.
The weekly holy day for Jews is called Shabbat. Shabbat starts Friday at sundown and ends Saturday at sundown.
The week culminates in the seventh day, the Holy Shabbat (shabbat kodesh, abbreviated ).
They prepare their children for Shabbat and light the Shabbat candles.
"Shabbat Shalom!"
The Jews eat the Passover Seder meal on the night of Passover (Pesach). It makes little difference whether Passover begins on Shabbat or on a weekday. Note that there are festive meals on every Shabbat, but they are not called "seder" and the foods are different.
After Shabbat, there is the Havdalah service.