You can book at the 180 day mark like other ADRs, but OFTEN/sometimes the dessert party spots are not "loaded" then. Unfortunately, if they are not available for booking at 180 then you'll just have to keep checking back as it's impossible to say when they will be added.
The Tomorrowland Terrace version is tougher to get as the inventory of seats/tables is very small. The Plaza Garden viewing version has tended to be more forgiving in terms of availability, even pretty close to the date of. The head count there is A LOT more given the size of both the dessert area and viewing area.
FWIW, I did the Plaza Garden Viewing in both May and a few weeks ago. Booked both in the 30 day or less timeframe.
I would bookmark the website and keep checking. i just did a pretend request for 2 people on July 22 and it is available. It will depend on your party size and the date you want. Parties are currently every night but in the fall that drops off to 3-4 nights per week. I would keep checking. That's what I'm doing for late January.
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