1. When the company or employer you work for, doesn't provide an annual Christmas party and an employee(s) throw it instead. "Unconventional" meaning you're invited and can invite your friends, family, co-workers, etc. and your friends can invite their friends, family, co-workers, etc. For some people, their jobs do not have Christmas parties anymore, or they are never asked to one, this remedies the lack of festive celebrations.

The general rule of thumb for attendance, once you are invited, you can invite anyone and then they can invite anyone. Someone should invite you before just showing up. Note: Everyone should pitch in similar to a potluck or a globally set amount to pay for a venue and food.

2. A Christmas Party hosted by anyone but the employer, primarily for their fellow employees. This Christmas party can include, but not limited to, their friends, families, significant other's co-workers and their friends and family.
John: Hey, my job can't afford to pay insurance to throw another Christmas party. Let's have our own "Unconventional Christmas Party!"

Jane: I'll try not to all go to jail for inappropriate behavior this time!

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Mark: Jannet just invited me to an "Unconventional Christmas Party."

Carol: What about your companies Christmas Party?

Mark: I'm skipping it because it's going to be lame! Nothing but upper management bragging about their year-end bonuses.
by Extreme Sen November 11, 2018
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Preston is the coolest guy ever. He hosts a Christmas party once every year where beings from all across time and space. Preston's Christmas Party can refer to the actual party itself, or it can refer to a bunch of people that shouldn't be in one place hanging out together.
I went to the bar and it was a Preston's Christmas Party.
by EESDSESESERDT April 7, 2022
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