What does "order of the day, the" mean?

The prevailing or expected mode, the current agenda, as in Volatility is the order of the day in high-tech stocks, or T-shirts and blue jeans were the order of the day for the picnic. This expression, dating from the late 1600s, originally alluded to the subject of debate in a legislature on a particular day, as well as to specific commands given to troops. Its figurative use dates from the second half of the 1700s.