Not sure if you are going on points, booking at the 7 month window, etc., but we were not able to get a room in the 7 month window for the first part of June this summer. We ended up going 4th of July and having a great time. It is difficult to get the first part of June because of the lower points. You definitely need to read up on all the ins and outs of booking online.
I think any time would be a great time at Aulani!!! Our 20th is next year as well, so congrats!