What is the best time to promote a player from the academy? At the beginning of the season or at the end? And what is the difference between them?

It depends on what you want to do with it!

Alqilouchi: What is the best time to promote a player from the academy? At the beginning of the season or at the end? And what is the difference between them?

Day1 your player got the minimum on stats, think for 17years is 1 everywhere
Day49 is the max he don't won more stats , think you have 15 points in advance something else
If you want keep the players for your team is better to store 7 training too