seasonal sports

When is it best to start dynamic training for seasonal sports

When you have built sufficient relative / maximal strength depending on the given sport.

I think he means how to program a year around progression from maximal strength to dynamic or endurance strength, and that way arrive at optimal strength level for that season in a given sport. As opposite to a normal training where you are focusing on maximal strength till you can move to dynamic. Please correct me if I´m wrong.

Others better suited will correct me if wrong but I thing you must focus on endurance strength or dynamic something like 3 months before competition but just guessing don´t trust me on this.

The vagueness of the question suggests that the OP needs to read some books on periodization. Thomas Kurz is an excellent author that comes to mind in "Science of Sports Training: How to Plan and Control Training for Peak Performance." Read that and come back with questions when you're done.

Sounds interesting. I'll read it!