Answer:
   
   The Sabbath/Shabbat/Shabbos/Shabbath is the seventh day of the week.
   It begins around sunset Friday night and ends around nightfall
   Saturday night. In some Western languages the word for "Saturday"
   means "The Sabbath", such as "el Sa'bado" in Spanish.
   
   The Sabbath commemorates G-d's refraining from continuing creation on
   account of the world being "complete." We emulate this by refraining
   from various categories of "creative work," such as starting a fire,
   building items, and ploughing fields, and lots of smaller-scale
   activities related to these. However, Sabbath meals are festive and
   song-filled, Torah learning is encouraged, and married couples are
   encouraged to reconsummate their union. The idea is to refrain from
   weekday activity in order to devote the day to sacred matters.
   
   The laws of Shabbat are extremely complicated, and the various
   movements differ considerably concerning which activities are
   permissible and which activities are encouraged. A popular thumbnail
   sketch is The Sabbath by Dayan I. Grunfeld.
   
   The Jewish Sabbath is on Saturday because of the passage in Torah
   (Bereishis [Genesis] 2:1-3) that describes how G-d rested on the
   seventh day and how we are commanded to similarly rest (see Sh'mot
   [Exodus] 16:28-30, 23:12, 31:12-17, and others).