Hi I need clarification for travel days activation.

I have read that you can modify a travel day only until the day before. Does it mean that, for example, if I want to leave on the 14th I must activate the travel day on the 13th? And If I haven't, I can't activate it on the 14th? 

Then about the train activation. Let's say that I must activate the day not after the 13th (in order to travel on the 14th), but can I activate the trains on the 14th? This in order to prevent myself from activating a train I won't take (if it's late or other reasons). Or if I want to decide on the same day I travel if I want to leave earlier or not, can I just activate the train on that day? Thanks