Exodus 20

8 Remember the sabbath day, to keep it holy. 

9 Six days shalt thou labour, and do all thy work: 

10 but the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: 

11 for in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it


Here God gave 6 literal days for the Jews to work, as he took this time in his creation process.

If these were not 24 hour periods, then the Jews would have a very strange week.