They submit their choices for Summer Training and their ranking (among the 1,150 or so in their class) in the Military Order of Merit and Academic Order of Merit plays an important role in which choices they receive.
There are lots of training blocks available, so most get something in which they have an interest. One thing is sure, there will be some hurry-up-and-wait for a lot of them right after the academic year is completed. Many will have what they think is a "set schedule", only to find that the ship or submarine that they are supposed to meet, won't be there at the specified date. Sometimes the ship will end up in Greece, rather than Spain on that date. Their travel isn't usually set until after they have their brief, which could be at the end of May (Block 1 begins 5/26), or in June (Block 2 begins 6/22) or July (Block 3 begins 7/20). Travel dates to land-based training within the United States are usually more of a sure thing - since wherever they are going - isn't moving.
So they may leave for training right after classes end, or they may have a couple of weeks off first. They may also have a week off between blocks. It can be a very fluid Summer.
In other words, don't make any plans that aren't refundable.
As for Summer Seminar or Plebe Summer, are you asking about someone who is a Youngster now, and wants to be a Detailer between 3c and 2C year, or someone who is a Plebe now and wants to be a Detailer next Summer before their Youngster AC year? I think if it's the latter, you'll find a precious few 4C Mids (after Plebe year) who will be selected for Summer Seminar, but I have not seen any Detailers during Plebe Summer who were selected from the 4C pool. Maybe I'm crazy, so someone will probably set me straight if I am.