Before I actually start, huge spoiler warning for the prologue of the show. I've tried to take out certain details where I can but this pretty much describes the entire second half of it.
Anyway, the happy birthday song is a bit of a dark reference in witch from Mercury.
Now, exactly who the child in the meme is would be way too big of a spoiler, so she'll just stay un-named.
In that scene her mother is trying to save her from a facility that's under attack because what they were working on there was deemed too unsafe, so she took her daughter and a prototype mobile suit to get away
However... some of the attacking mobile suits caught them, specifically one that can disable the ability of the prototype suits.
So, her dad, in the most badass sacrifice in the show, tackles the attacking mobile suit with a less advanced prototype and pushes his beyond what it can disable which put his life in extreme danger
... Now here's why it's a dark reference
I didn't mention before, but this entire attack happened on the kid's birthday.
So, the dad, in his dying breaths sings what he can of happy birthday to his daughter. The image in the meme is from mere seconds after he died.
This isn't even the only time a guy died while someone sang happy birthday either, it happened again later on
Caption this Meme
