Power Query M code
let
Source = YourDateTable,
AddedIsWeekend = Table.AddColumn(Source, "IsWeekend", each Date.DayOfWeek([Date], Day.Monday) >= 5, type logical),
AddedIsWorkingDay = Table.AddColumn(AddedIsWeekend, "IsWorkingDay", each not [IsWeekend], type logical)
in
AddedIsWorkingDay