It's usually in the last 2 months. My husband grad. in May and we found out in early April where we were headed. We had an idea and those at the top of the class 1-5 possibly knew they were going to subs if they wanted to. We had 5 sub slots open, but only 4 or something wanted subs. If I remember correctly we put down our first 6-8 choices and then they chose for you. There are usually the same amt. of jobs/options as there are people in the class. There was also one toss up. One guy chose that and he was surprised at the reading. It was fun and nerve wracking.
Hope I make sense and that this helps.