Watching The Residence and tracking the details like I love to do, but this post is still about Stranger Things.
I realized that had Stranger Things been a condensed series/released all at once - or even had I just personally first watched it with at least 4 seasons out already and the knowledge I have now
There would be no doubt in my mind that Will Byers' birthday would become pertinent from the moment it was mentioned. Because it did not need to be.
Let's go through the shed:
the rainbow ship - queercoding (and speculation relative to the multicolor rocket play structure across from the Creel house but that is yet to be seen)
should I stay or should I go - music is later revealed to specifically help people out of the state he was in.
Castle Byers - adding emotional weight to its future destruction
the day Mike and Will met - romantic buildup
Mike's D&D story - references "tunnels" like the ones under Hawkins in that season
Will's birthday - ......nothing? Just the date?
They could have said "do you remember your 6th birthday party?" or whatever it was and jumped straight into the story, just like Mike and Jonathan did - starting with "do you remember...?" but she didn't. She stated the exact date. The purpose was emotional memory, no longer a genuine test to his memory, so that was, for the character, emotional emphasis, but for us, *important information to be saved for later*.
Writers are taught early on to cut any line unnecessary. If you can do it faster, do. I great writer does it one line. An amazing one does it in none. Of course, you do also need them because people do talk, but to not cut - or even write - "Do you know what March 22 is? It's your birthday. Your birthday" is a waste of screentime, a waste of page space, a waste of digital storage and a waste of table read time - even if by only a few seconds, it's cut because those things add up if they're in every scene and they add up quick.
No, it mattered. They stated an outright fact and just left it there. They had the actor play it very emotionally to make it seem like it was justified by the moment. It wasn't. They said stated a fact. No different than The Residence saying "look at that incinerator, anyways..." it's just a fact. Not a throwaway line to fill conversation - because nobody even really just states objective truths to fill time, so that wouldn't be how you'd do that. A fact. And when a show with any level of mystery gives you a seemingly unimportant fact, it's a setup. It might be a long-term setup, you might forget.
Maybe you even thought it was a setup but the season ended, and the series as it existed, and you forgot about it and considered any setups in previous seasons null and void for future ones. You would be wrong. I would be wrong, I have done this plenty. But going into the show as a single work and hearing "Do you know what March 22 is? It's your birthday. Your birthday." I would go "they're telling me that".
And yes, I'd probably forget it by the time they put "March 22" on the camera screen, but need I also remind you that screen did not need be shown in the first place. Again, a waste of editing time if it was pointless. We were shown a shot of the boy filming, we didn't need confirmation that the camera was on. But the only visual medium to show us the date at Rink o Mania was through a camera lens. There are no smartphones. There are no calendars at roller rinks. And walls aren't visible when shooting a walk down an airport hallways. But we needed to know that it was March 22 without being expected to hold onto it and do the math of "she was arrested on March 23, -1 is his birthday", they needed us to know in the moment.
Honestly, they didn't even need the camera! They never showed the tape to anybody, which I kind of expected because I understood that it had to be some sort of information for later, just not what. But they just watched it back and laughed. They could have been talking and laughing when El caught them. They included the camera exclusively to find a loophole to showing you the date in a random public place. And they wrote into season 2 the objective statement "Do you know what March 22 is? It's your birthday", something no part of the scene or following lines was dependent on. If not informational, it could have been cut.
If his birthday does not mean anything, we shouldn't know when it is. And had it not already been applied, El's bullying would not have been filmed. The cops didn't watch the video, it wasn't shared with kids at school, none of them took efforts to destroy the tape as part of that plotline.
It isn't about them slipping up with the wrong date. It shouldn't have been a slip up. Because we shouldn't have even known what March 22 was.