As soon as the TV companies have made their choice.
One would assume that Man Utd v Spurs would be seen as the most commercially attractive so what would that mean? Saturday evening for them and Sunday for us?
'If the FA Cup winners finish in the top five of the Premier League, their UEL group stage place will go to the next-highest ranked team not qualified for UEFA competitions in the Premier League.'
https://www.premierleague.com/european-qualification-explained
Yes, any suitable neutral ground was chosen. Consideration was given to the facilities, capacity, location.
Let’s not forget the tragedy at Hillsborough which had regularly been used as a venue for semi-finals until then.