Sunday. Because Xiao Ming only exercises from Monday to Friday, and the number of days of exercise in each quarter is the same, that is, the number of working days in the second, third and fourth quarters should be the same. Among them, 9 1 ÷ 7 =13,92 ÷ 7 =13 ...1,which means that1day in the third and fourth quarters must be a weekend, Saturday in the third quarter and Sunday in the fourth quarter.