Translation:

3. We sent it down during a blessed night: for We (ever) wish to warn (against Evil).

Notes (Tafseer)

4690. Usually taken to be a night in the month of Ramadhan, say the 23rd, 25th, or 27th night of that month. It is referred to as the Night of Power in xcvii. 1-2. See also ii. 185. But perhaps we need not fix it literally by the calendar. The night that a Message descends from Allah is indeed a blessed night like a day of rain for a parched land.