"Apply early" doesn't mean a year in advance. Getting your resume in early may have benefits. But, don't go overboard, applying so soon that your job application is lost or completely rejected.
Here's the official word from Kay Lynn Odle-Moore, the Volunteer Coordinator for Tetlin National Wildlife Refuge: "Positions are competitive and fill early, so if you're interested in volunteering for the summer of 2012, please get your applications in early. We will begin accepting applications in August of 2011 for the summer of 2012."
Note that she stresses "early." But, then she defines that. If summer positions start in May, she's saying apply no more than nine months before the start date.
Certainly, agencies that use volunteer workers like knowing they have a full volunteer staff lined up well before the season begins. They can breathe a sigh of relief when they have the positions filled with qualified helpers. But, experienced volunteer coordinators know that those who commit to a position too soon do not always show up to fulfill their assignment.
With many seasonal businesses, three to six months prior to the starting date of the job is more appropriate.
You may want to apply early for a position, but don't go overboard. I advise against applying more than one year in advance of the position start date. The exception, the reason you would apply earlier, is if an organization places an ad specifying earlier applications.
If applying early gives you a sense of security, go for it. However, jobs -- both paid and volunteer -- are available with immediate start dates. In some instances, applying early may mean you have a choice of locations and work duties. The main benefit to getting your application in early is probably not to beat out the competition, though, but to give yourself peace of mind.