When Do Most Colleges Update Their Undergraduate Admissions Application For The New School Year?

I’m currently a junior and I’m trying to get a head start on college applications (I know it’s way too early but I want to do start as early as possible so I don’t feel rushed). When will most schools update their undergraduate admissions application? In this case, for Fall 2019.

Also, when does the common app update their essay questions, and do they leak them before they do like I saw on an article?

This is indeed way too early. Common app for the new admission cycle usually opens on Aug. 1 and it almost never changes essay questions. Basically, you can write anything you want within 650 words. What could change are supplemental essay questions from different schools and those are usually released between Aug 1 and 15th. I would say nine out ten times supplemental questions stay the same too. So, if you are really desperate to get a head start you can open a common app account and start thinking and practicing on it.