When does Taizong want his officials to train the people for war? in the springtime when they are not busy farming during peacetime after war has begun

Li shimin, the second emperor of Taizung empire was an ardent scholar of confuciusm. He taught that people should be taught how to fight in war when they are not busy fighting. he referenced confuciusm which articulates that taking untrained people to war is throwing them away.