Watson's Biblical & Theological Dictionary 
A period of seven days, under the usual name of a week, שבעת , is mentioned as far back as the time of the deluge, Genesis 7:4; Genesis 7:10; Genesis 8:10; Genesis 8:12; Genesis 29:27-28 . It must, therefore, be considered a very ancient division of time, especially as the various nations among whom it has been noticed, for instance, the Nigri in Africa, appear to have received it from the sons of Noah. The enumeration of the days of the week commenced at Sunday. Saturday was last or seventh, and was the Hebrew Sabbath, or day of rest. The Egyptians gave to the days of the week the same names that they assigned to the planets. From the circumstance that the Sabbath was the principal day of the week, the whole period of seven days was likewise called שפת , in Syriac שבתא , in the New Testament σαββατον and σαββατα . The Jews, accordingly, in designating the successive days of the week, were accustomed to say, the first day of the Sabbath, that is, of the week; the second day of the Sabbath, that is, Sunday, Monday, &c, Mark 16:2; Mark 16:9; Luke 24:1; John 1:19 . In addition to the week of days, the Jews had three other seasons, denominated weeks, Leviticus 25:1-17; Deuteronomy 16:9-10 :
1. The week of weeks. It was a period of seven weeks or forty-nine days, which was succeeded on the fiftieth day by the feast of pentecost, πεντηκοστη , "fifty," Deuteronomy 16:9-10 .
2. The week of years. This was a period of seven years, during the last of which the land remained untilled, and the people enjoyed a Sabbath or season of rest.
3. The week of seven sabbatical years. It was a period of forty-nine years, and was succeeded by the year of jubilee, Leviticus 25:1-22; Leviticus 26:34 . See Year .
American Tract Society Bible Dictionary 
Or successive periods of seven days each, were known from the earliest times among nations remote from each other in Europe, Asia, and Africa, Genesis 29:27 . See Sabbath .
The Hebrew had only numeral names for the days of the week, excepting the Sabbath; the names now current among us being borrowed from Saxon mythology. The Jews called Sunday "one of the Sabbath." A prophetic week and a week of years were each seven years; and a week of sabbatical years, or fortynine years, brought round the year of jubilee. In John 20:26 , the disciples are said to have met again after "eight days," that is, evidently after a week, on the eighth day after our Lord's resurrection. See Three .
For the "Feast of Weeks," see Pentecost .