The first Sunday. Sunday is your day for ironing, shinning your boots, and if there is time, write a quick letter. It may be different now. That is how it was in 99'.
Pretty much whenever they want. Before they ship downrange to their BCT/OSUT unit, they'll be in the Adjutant General's battalion, and might not write during that time since they'll only be in the AG...
Whatever you feel like writing. If you're sending a postcard, where the writing is openly visible, you would want to use discretion in what you wrote on it. Or, if you wanted him to suffer it out a...
Training consists of learning military ranks, shipboard firefighting, swimming, attention to detail, taking and giving orders, teamwork, physical fitness, and general naval knowledge and etiquette....