Your choice, really. Having a save you're enjoying and have an extended period of time in-game pre-booked is a great start. You can write what you already have booked, which means you have 5 or 6 months guaranteed of shows which is longer than many diaries (including my own from the past) go and would give you a foundation to continue building upon. You could also start writing from the point you're currently at and summarize what's happened up to that point.