Generally, if they are on OSCAR, they will take the applications on a specific date. I did it last year, and I think the date is around the beginning of October. They will then contact applicants a week or two later. I can't remember the date, but I got a call from one judge literally 5 minutes after they could begin contacting students.
That being said, not all judges follow OSCAR. Some have their own time table. If the judge is on OSCAR, then they will likely follow it. However, if they're not then they will probably take it ahead of your 4th semester grades. Be advised that if the judge is on OSCAR, then you need to make sure you have all your apps in on the date that its due. You don't want to miss out.
If you don't see a judge on OSCAR, and you're interested in them, I suggest you call their chambers and ask about what their process is.