It should start when you use it in April. If you are not staying on-site in April then there would be a small advantage to buying a park hopper and then upgrading that ticket to an AP, though, in that you could get a Magic Morning.
Normally I would actually just say to wait to buy your AP until you get there, but Disney has been hiking the prices of APs lately, so buying it earlier MIGHT (but probably won't) save you $10 or $20 per ticket.
That said, you can also volunteer with the "Give a Day" promotion to save $72 off of your AP (the value of a one-day, one-park pass, that can be upgraded). And you could even get Magic Morning if you volunteered, upgraded your one-day ticket to a 3+ day hopper at the gate, and then upgraded THAT to an AP after you used your Magic Morning. But this will not work if you buy your AP before arrival.
Do you have to confirm any info from your AP to make your June reservation? I thought that with most reservations you can say you're an AP-holder and then get the discount when you show it at check-in. I know HoJo's works like that and I'd HEARD the Disney hotels did too but don't quote me on that.
Have a great couple trips!
