Festivus is a day when everyone becomes friendlier for some reason. I take it is because of the gifts everyone wants to give each other. Nothing says that you care more than a five-thousand gold watch. A part of Festivus is hanging out with those you love and enjoying the, somewhat brief, winter. Sometimes people lose the point and give people tangible gifts that mean something to them, but not to other people. Sure, that watch is worth a lot, but not to someone who doesn't need a watch.
As you know this is Oshunmare, not Earth. A pizza, not an orange. It was birthed through a harsh winter where people were barricading themselves. Some were strangers and didn't feel at all comfortable with the predicament. Soon, those inside pass the time, get to know each other, and bond over simple things. It got to a point where they gave makeshift gifts to those who feel left out. Blatantly, it was stealing, but who cares. Now, Festivus is commercialized by deities and corporations. Not many people celebrate to enjoy their own bretheren, but those who do cherish the holiday.
Well, obviously you have to be in a room with people. then you give a gift of two types. Either: you give to a friend or a gift to everyone in the room. Though this is a party birthed from a pseudo-apocalypse; so sweets, drinks, and roasted meat are in order to entertain guests. Sometimes games are involved, one most obvious is combat on who can be the Champion of Festivus. It's just a pass time, it's like your average competition where the winner gets to sit atop a throne of hard light and cut the first roast for their household. Nothing too serious.
This little holiday is one of the holidays everyone celebrates, which doesn't say much but it does say something like 'How we can all get along and not be in constant war with each other.' Or something along those lines.