To be serene is a very nice state of mind body to be in. Free of negativity full of compassion and love for all. Free of anxiety and pain. Feeling freee in your spirit and soul.
To be tranquil in a peaceful state of mind. The trick is to stay that way wehn everything is going wonky. I have found it after hours of meditation. I can now achieve the state of serenity without repeating those hors of meditation.
As I look at it I would say that it comes when I go into observer mode. By this I mean I observe without attachment or judgement. To witness event just as they are without projection of my egoic programing or emotions..
I am not saying that I am free of attachment and emotions just that I can enter this state of serenity they subside into the background of my mind. At least when in meditation or observer mode. I would suggest that you give it a try to achieve this serenity just take a change in consciousness. Or perhaps it is simply a desire to be serene.