When one applies for NROTC (I know this is true for Navy but I'm not sure about Marine option), they select their desired job (for lack of a better word) in the Navy upon commission (submarines, special warfare, aviation, etc.) They are also asked to write a short essay about why they selected...
This year CONTRAMID dates are May 17 to June 17, 2017 (West) and July 12 to August 12, 2017 (East). Fortunately DD wedding falls in between both sessions so DS will be able to attend.