We cruised during Christas, and the HMC cabanas did not go on sale until 3 weeks prior to the cruise date. I the past I have been able to book them a year out.
I just got off the Spirit last week, and the HMC cabanas were not listed until the week before the cruise.
I booked the cruise in July and checked the excursions page at least twice a month up until November when I started checking every day. Finally, 1 week before the cruise they showed up.