They will be released once they confirm the timetable, I will however say that Avanti are a complete and utter shambles, rumours are the franchise won't be theirs much longer.
I live up north, I needed to get back on a Sunday and they released the tickets the Friday evening before. Took the risk and it worked out, it’s safe to say they’ll have trains running as long as there are no strikes, but yes it’s very stressful.