You'd be surprised by how many people fail to submit their applications on time. The first grad school deadlines are in November and extend to March and beyond. While some schools will review late applications, it's a bad idea to start on the wrong foot. Best practice calls for submitting your applications a couple weeks in advance of deadlines, so that if anything is missing, you have time to make amends.
But why do so many people fail to submit everything on time? A few reasons. Chief among them is the fact that GRE scores take up to 4 weeks to reach schools. That comes as a surprise to many people, and self-reporting your score is usually not good enough. Another reason is the time it takes mail to travel. Some schools require hard copies of applications and other material such as your undergraduate transcripts. Add to this the time it takes for your undergraduate school to send out the transcript and all of a sudden you've missed your deadline.
The key is to plan. Plan, plan, plan! Below is a timeline of how you might plan your calendar if your first application was due by the end of November. It assumes that you're aiming for a high GRE score and that you need to produce a writing sample from scratch. If you've already taken care of some of these requirements, then you'll need less time. But don't neglect identifying all the grad school deadlines and working your way back to plan what you need to accomplish by what date.
Graduate School Timeline*
* Assumes grad school deadline is November 30th.