As PP indicated you do pay when you reserve, but there is a 48-hr cancellation policy so no problem if you cancel or change your plans as long as you do it at least 2 days prior.
We booked the Luau for April 24th (our 25th wedding anniversary!) on November 11th so greater than 5 months ahead. However, it doesn't appear that it's on a consistent schedule like WDW which is always 180 days for most restaurants and dinner shows. When I first checked a few weeks earlier there were only bookable dates maybe 3 months out and then suddenly a couple more months of availability appeared so it seems they may release dates in 'chunks.' Also be advised that the Luau only occurs on certain nights -- we booked Tuesday on our actual anniversary but the luau is also offered on Wednesday, Friday and Saturday and right now it's bookable thru August so that's 6 months.
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