When I got my first FS last year it just said you have to report on (Insert Date). You are joinging an elite yada yada congratulations with more yada yada...start a physical training program.
OCS website has recommendations on when to arrive but I think it would be a good idea to be there a day or two early so that you have time to rest, settle down, focus and maybe meet your fellow OCS mates.
I will be awaiting the next FS letter here soon and I am anticipating being in this class due to my age but who knows with Big Blue.