I want to warn you that if you choose a popular venue for your ceremony or reception, that is where you'll have to make the most compromises. In all likelihood the most popular venues will already be booked. So, if you have your heart set on a specific venue, it's best to plan as far in advance as you can. But if you're free & easy and willing to go with the flow, your planning will be easier.
Please understand that Disney does have a minimum amount of time that an event can be planned within. They won't waive from that. (If you ask them what it is, they will tell you.) Someone here a year or two ago wanted to pull off a wedding with just two weeks (I think) notice. Disney wouldn't do it.