when is freeplay? (The Witcher 3: Wild Hunt)

I apologize, but I did not find the topic "questions from newbies", so

I create. I got to the quest "Isle of Mists", passed it, ended up in Kaer Morhen, and could not move to another location.
So the question is:
After what story quest will freeplay begin? After the battle, or so until the very end, will they not be released?
I would just like to complete some side quests before they turn gray