If you need to pt players you already got, best time to do so is last 2 season days. This way your players will progress more since their age will get bigger next season and aged players will train a bit less than you expect em to.
If you are going to bring new players in next season, you have to pt at first days of season, preferably before league starts.

Power training is a nice tactic but you need to pick the right time for it!