Ya tbh I'm not really sure I get missing the game for a birthday party. I can't recall a similar example of a player missing a game because of a kid's birthday, which kind of makes me think that's not what this actually is.
If it was I'm not going to be angry about it, people have taken time off for much dumber reasons myself included. But we should probably actually get this confirmed before we make too many judgements, the birthday might just be coincidental timing and some family member may be sick or something.
If it was just a maintenance day they'd have listed him with some minor ailment, also they wouldn't do it before 2 days off, they'd do it on a B2B or something.