If I remember correctly, the dates of MNSSHP came out in late April or early May, but of course that could change. The last time I bought tickets was 2009.
You can often figure out which dates are likely to have a party when the calendar for September and October are released. Look for nights when the MK closes at 7:00 pm. Those usually turn out to be the party dates. September's travel agent calendar should come out in late-February/early-March (six months) and travel agent October's calendar should come out in late-March/early-April.
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