1] the seventh day of each week which was a sacred festival on which the Israelites were required to abstain from all work
   1.a] the institution of the sabbath, the law for keeping holy every seventh day of the week
   1.b] a single sabbath, sabbath day
2] seven days, a week
derivation: of Hebrew origin (07676)
http://bible.crosswalk.com/Lexicons/Greek/grk.cgi?number=4521&version=kjv

(07676)
1] Sabbath
   1.a] sabbath
   1.b] day of atonement
   1.c] sabbath year
   1.d] week
   1.e] produce (in sabbath year)
derivation: intensive from (07673)
http://bible.crosswalk.com/Lexicons/Hebrew/heb.cgi?number=7676&version=kjv

(07673)
1] to cease, desist, rest
   1.a] (Qal)
       1.a.1] to cease
       1.a.2] to rest, desist (from labour)
   1.b] (Niphal) to cease
   1.c] (Hiphil)
       1.c.1] to cause to cease, put an end to
       1.c.2] to exterminate, destroy
       1.c.3] to cause to desist from
       1.c.4] to remove
       1.c.5] to cause to fail
2] (Qal) to keep or observe the sabbath
http://bible.crosswalk.com/Lexicons/Hebrew/heb.cgi?number=7673&version=kjv

Additional information:
1] See the details of the peculiarities of the Jewish way of measuring and counting days.
2] The Sabbath is the day on which the Shewbread is offered on its altar.