The Day the Gates Opened was the day that the one and only God that had access to the planet decided that the planets isolation was no longer necessary.
This was reasoned in 2 ways. One, the initial reason of the isolation proved to be false. The disease the planet had (lack of magic) is no longer catching. The other planes would not be in danger of becoming magicless if they came in contact with the Grey Dragon World. This was the result of the true healing the “Individual” had been casting for the last 3000 years.
The second reason was that the races of humans were ready to come into contact with other worlds.
What happened on and after the day ?
The Mythral that blocked gates to form was dispelled by its creator the Grey Dragon. This changed the world in a visible way. The starlight became brighter and more blue, the sun’s color became less yellow and 2% stronger. In the first decade of this change several large scale weather events plagued the seas and the coastlines, several large scale droughts lead to immense forest-fires and changes in crop yields. Deserts grew and icecaps retreated.