That day is then a harsh day;
That will be a difficult day.
that shall be a Harsh Da
That will be -- that day -- difficult day
So that, that day (is) a difficult/distressing day/time
That will be a very somber day
So that is a tough day
That day will be a hard day
So that Day (the Day of Resurrection) will be an extremely hard day
That will be a day full of woe and distress (for the opponents of Truth for this day will spell final defeat of disbelief)
Truly, that Day will be a Hard Day