Excuse my ignorance: One year from the day I walk through the gate at Magic Kingdom on June 16th? So when I link them to my account later tonight to book my Fastpasses, they will NOT be activated?
Nope. When you buy an AP online, it's a "certificate" - you can use it to book FP+ but it won't let you enter a park until it's activated at a ticket booth or Guest Relations. That's when the pass year begins. (Even if you activate the pass and don't enter a park right away.)