As someone who’s stayed at Aulani during both time periods, I really enjoyed visiting during the Christmas/New Year’s holiday.
Last year, I spent Dec 20-Jan 2 in Hawaii. I stayed at Aulani until a couple of days after Christmas and then flew to Maui. It was my first time being in Hawaii for Christmas/New Year’s and had been on my bucket list for a while. I enjoyed going to Waikiki to see Honolulu Hale, Oahu’s Christmas decorations. Aulani was full, but it wasn’t any fuller than it was in June/July when I’ve traveled previously. Just be sure to pack your patience because some things take longer.
In December, the weather on Oahu was mainly in the 80s with some days of rain. However, that didn’t keep me from doing any of my planned activities which I pre-booked. Remember, when it’s raining on one side of the island the other side usually has sun. I visited the North Shore, Waimea Valley, Kualoa Ranch, Byodo-in-Temple, took paddle board lessons, went shark diving, and took a helicopter tour among other things. At Aulani, I saved a couple of days to sit by the pool. Yes, they were crowded, but the adult pool had a lot of chairs available. The water was a little cool, but felt good after sun bathing. Aulani also had Santa Stitch on Christmas Eve for people to take pictures with. There was a special Christmas dinner on/near the beach you could make reservations for but it was very expensive and sold out early. You could also order Christmas dinner for your room that served four people. The resort had three different Christmas trees and garland, but that was it for decorations. I also did the luau and really enjoyed that. If you like football, the Aloha Bowl is played on Christmas Eve. I thought about going but ultimately decided against it since I had the luau that night.
I loved being on Maui for NYE. The weather was sunny everyday and I did snorkeling and whale watching tours. I did the Drums of the Pacific Luau on NYE and was able to see some of the fireworks from Lahaina. Unfortunately, the Maui airport was insane and really crowded coming and going.
The only difference in visiting during June/July is that there are typically more sunny days. The pools are still crowded. In general, Aulani has gained in popularity and you should count on the resort being busy. If you visit over July 4th, the Ko Olina resorts usually do fireworks in the lagoon which you can see from Aulani beach.
Have fun when you go in June. I’ve already booked my second Christmas/New Year’s trip for Hawaii for next year. If you’re ok with visiting Hawaii in June, visiting in December isn’t that different.