From the article, here's the edit: It's a simple mistake of an expansive series costing $30 million an episode, really: the creative team set an episode of Stranger Things 4 on March 22, forgetting that it had been established as Will Byers' (Noah Schnapp) birthday in the earlier season.
So can someone explain to me why this is a plot hole? If the episode took place on March 22nd, Will Byers' birthday, we would expect... a birthday party?