How do we know when the sabbath is?

Nowhere in the Bible will you find a passage that specifically addresses working on Sunday. The scriptures in both the Old and New Testaments that address a day of not working reference "the Sabbath." The Bible defines the Sabbath in Ex. 20:9-10, saying that people should work for six days and then rest on the seventh day. The Jews observe the Sabbath on Saturdays, but the Christians moved their Sabbath to Sundays in recognition of Sunday being the day of Jesus' resurrection. Neither Saturday nor Sunday is specifically defined as THE day to observe the Sabbath in the Bible.