Summer between Junior and Senior year. In the meantime, make up a list of everything you've done since starting high school - every award, every volunteer day, everything you participate in. We had one, and the people that wrote DS recommendations loved it - it reminded his teachers and references what he had done and allowed them to comment on those things they knew best. (For instance, his English teacher is also the faculty advisor for Quiz Bowl.) When we requested the recommendations he did it in person, but followed up with an email with the list attached. And that's all it was - a list, not a formal CV with details.
By the time the dust settled, he had filled out applications for two service academies, three ROTC programs, three nomination sources and four colleges. That's a lot of essays to write. Good luck!