When is the best time for an athlete to attempt to optimize weight and change body composition?

Offseason period.

Explanation:

Offseason time or period for an athlete is the time when an athlete does not have or engage in any competitive events. it is the period of rest,at the same time, the period when the athlete is preparing for the upcoming season.

This time usually falls at the end of the year. At this time, the athlete will work on his weight to make it good and effective as possible, so that he will be in his best form when the season starts.