As an educator(tutor), I believe the best time would be non-school days, holidays. Without school, CCA etc, so much time has been freed up. Students can then proceed to do self-learning at their own pace. Tuors are like a regular checkpoint every week. I would prefer parents to cramp more tuition lessons during the holidays, rather than right before PSLE / GCE O Levels / GCE A Levels. As I tell my students who have been putting in efforts for Os and As, stop my tuition and go focus more on other subjects which they struggle as I am confident that they can score the highest grade already. No difference between a 75% and a 100% for A1 right?