I've been looking into this for the last few days. Today marks 60 days until Thanksgiving on November 24.
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Disneyland Dining CM informed me that technically, reservations should open up for Thanksgiving Day today, 3 AM Pacific Time for the website and 7 AM Pacific Time by telephone. However, they were still in the process of loading seating inventory into their system so reservations could be delayed until Tuesday, September 27.
I checked the Disneyland Dining website and called their phone line this morning, and reservations for the Thanksgiving Feast in the Disneyland Hotel are still not available (although Thanksgiving Day reservations for Disneyland Park, Disney California Adventure and Downtown Disney table service restaurants, and Steakhouse 55 are up).
The CM on the phone told me that reservations for the Thanksgiving Feast in the DLH will start on Tuesday, September 27, 3 AM Pacific Time on the website and 7 AM by telephone.
We've been doing the Thanksgiving Feast in the Disneyland Hotel every year since 2010 (but took a break last year for a trip to WDW), and based on experience, reservations will fill up or very nearly fill up on the first day, with a lot of time slots disappearing within the first few hours. The ballroom in which this buffet is held is very large, but this is a popular holiday event and for a lot of people, like my family, it's an annual tradition we try not to miss. I don't know how US Pacific Time translates to your local time in Australia, but I would find out so I can get on the website as soon as the reservations are supposed to be available. The reservation does require a credit card to hold the reservation but will only charge if you do not cancel in time and don't show up. I believe in the past, because this is a special event, cancellations had to be made within 2 or 3 days prior to the event (not your typical 1-day before).
The Thanksgiving Feast is a really unique dining event at Disneyland and definitely fun! I don't think the resort puts on anything else so grand during the year (the only smaller-scale comparison I can think of might be Mahaloween). The food is good, plentiful and traditional holiday fare, you'll find food and dessert stations throughout the ballroom (and kid's stations too). There's a live band that plays Disney and holiday classics. There are several character meet-and-greet stations set against seasonal backdrops throughout the ballroom, including characters like Pocahontas and Meeko, Pinocchio and Geppeto, Chip and Dale, Koda and Kenai (although in 2014, I think they were replaced by Robin Hood and Friar Tuck). Also in 2014, Minnie showed up to play host with her own dinner table for the first time in the many years we've attended (I believe Donald, Daisy, and Pluto were also at her table). The characters rotate around in pairs and there are character attendants who can take your pictures with them. In the past, the check-in process could be a little long and trying on our patience (and hunger), but the fact that we attend this event year-after-year and will do it again this year should tell you it's something really special on our holiday calendar.