What is the deciding momento? What i mean is which update should i say no to a date to have this scene with Cassius and the horse? Nothing in the game gives you pointers for thisThe only thing is, I assume if you choose to play a couple, or even all of the dates. You'll then have to play them as separate playthroughs. Because there's bound to be more dates in the future, which will be dependent on you playing the first date. So everytime there's an update you'll have to play all the content on each of those playthroughs. So you can eventually play the next set of dates.
And just hope Zorlun doesn't make some major change in the future, that requires you to start again.