If you own Vistana and are trading Staroptions, the window opens 8 months out. I have been able to reserve weeks even for high demand dates calling at exactly midnight Eastern Time 8 months out. Sometimes the site is unstable, but trying 15-20 minutes in has gotten me a reservation.
Just a couple of days ago I reserved 23 nights at the Westin Ka'anapali Ocean Resort Villas for arrival mid April (about 1 month ahead). I first reserved the maximum 21 nights allowed by the web site, then made a second reservation for 2 extra nights. There was not much selection with respect to unit sizes and views, but I wanted an Island View Studio and got it. I had a similar experience a few years ago when I decided to go to Maui on a Wednesday and found a reservation for the following Saturday. This was in January, so not the highest demand.
I have never traded into Hawaii using Interval International.