For me, it was 6yo DS sighing contentedly when MIL and BIL arrived 20mins after my parents stayed and saying "now everyone is here and they'll be here for ages and ages". Yes, DS, they will be here until Thursday which isn't what I'd necessarily chose but comments like that make me realise why I do chose to do Xmas like this.
And then seeing DMum and MIL, who have nothing in common, giggling together over something after a bit too much champagne before lunch. They each hosted Xmas for decades and both had elderly relatives to ferry home at the end of the day so never had the chance to have a mid-day drink so I always enjoy seeing them having fun and now make cocktails that I know they will like and may not fully explain quite how potent some of the ingredients are