The date of the night of decree is not mentioned in the Quran and it is just speculation.
Also, the Quran refers to this as the night/period the Quran was first revealed and it is not correct to treat this as the winter solstace, or the date of any other decree as this ties Allah to one day a year to do His bidding which is a jewish custom, not Quranic.
97:1 Indeed, We have revealed it in the Night of Majesty. [44:3]
97:2 Ah, what will enlighten you what it is, the Night of Majesty!
97:3 The Night of Majesty is better than a thousand months. [A day of enlightenment is better than a life-time of ignorance]
97:4 The universal forces and the Divine revelation have descended therein, by their Lord’s Leave and shall work in concert with every decree to carry out His plan.
97:5 Peace! It is a message of Peace and Security, and inevitably, a new Morning of Enlightenment shall dawn. [39:69]
48:15 If you go forth to a battle that holds promise of spoils of war, those who had stayed behind before, will say, “Allow us to follow you.” They seek to change the verdict of God. Say, “You shall by no means go with us. This is what God has decreed already.” Then they will say, “You are envious of us.” Nay, but very little it is that they understand (of selfless service). [Battle is no longer a means of winning booty as it was in the Age of Ignorance. It is permitted only against persecution and in self-defense. 2:167, 2:193, 3:140, 8:1, 60:8-9]