It is always important to choose a time to plan.
There are some key facts of life about planning that are unique to teaching. The first is the uniquely distorted pattern of the school year. Choosing a time to plan is not easy - here is how one teacher summarised the issue.
Middle of Summer vacation: we have lots of energy, lots of spare time, but little information about the following year's timetable
Start of Autumn term: we have lots of energy, no spare time, much more information available
Start of Xmas vacation: we have no energy, a little spare time, adequate information about classes, lessons, workload
Start of Spring term: some energy, no spare time, plenty of information about everything
Start of Easter vacation: no energy, a little spare time, plenty of information...
… but it's really too late to be planning for this year anyway.
The practical consequence of all this is that if we are going to plan at the start of the academic year – and we certainly are - we need to plan with limited information. The plans that we come up with will not look like the plans that we end up using; they are deliberately designed to sketch out what is needed, to be flexible, to be easily altered and expanded as term progresses.