It's not a "strict" rule, and only applies to HSC assessment tasks.
In addition to the schedule of dates for assessment tasks, there must be provision for adequate notice of the precise timing of each assessment task. Generally, at least two weeks' notice of the details of a task should be given.
I say "not strict" in the sense that it states that you must be given "adequate notice" - most of the time two weeks+ is the minimum, but if determined that less time is still adequate for an assessment task then that is not forbidden.