Boyd’s Bible Dictionary:
The division of time into weeks of seven days each dates from the earliest historic times among many and wide-apart nations
The Hebrew week began on our Sunday, their Sabbath being the seventh day or Saturday. The only day of their week they named was the Sabbath. The rest ran by numbers, as first, second, third, and so forth. Besides their week of days, Hebrews had their week of years, every seven years, and their week of seven times seven years, or year of jubilee, every fiftieth year (Gen. 8:10; 29:27). The “feast of weeks” corresponded with Pentecost (Ex. 23:15; 34:22; Lev. 23:15-22; Num. 28).