Nachiketa goes to the house of Yama — the god of death — and waits three days without food or water. When Yama returns, he offers three boons in compensation. Two are conventional. The third makes the Katha Upanishad immortal: teach me what happens after death.
Not as a morbid question. As the question of a person who has been sitting with death for three days and has discovered that most of what he thought was important looks different from that proximity.
Marcus Aurelius's Memento Mori practice — remember that you will die — is the most efficient tool available for distinguishing what actually matters from what appears to matter in the context of unlimited time. Given unlimited time, everything can be deferred. Given a finite horizon, the distinction between urgent and important clarifies immediately.
The Buddhist practice of Maranasati — mindfulness of death — is one of the nine classical contemplation practices. The instruction: contemplate the fact of your own death in detail. Not as an intellectual exercise. The traditional report from practitioners is consistent: after genuine Maranasati, the things that were urgent this morning no longer appear urgent, and the things that were background appear foreground. The reordering is not manufactured. It is what happens when the assumption of unlimited time is temporarily removed.
The question requires a genuine, written answer. Five years. What would you stop doing? What would you start? Who would you spend time with that you currently see too rarely? What would you say that you currently leave unsaid?
Those answers are not instructions for after a terminal diagnosis. They are instructions for now.