There are none I'm afraid! Whenever M&A appear will be a suprise, sometimes they appear around special dates in the year, and sometimes when they feel like it. I'm not sure about christmas, I imagine they'll be celebrating with their family too, but maybe they'll have a free spot to jump in-game? But it's never announced. When they do appear though, usually someone will make a post on the community site so people who aren't in-game at the time can join too.