Middle Child's Day is a real holiday, though most don't know it. "You can barely find it, let alone the origins of it," Hopman said.
The founder and sole member of the IMCU wants middle children everywhere to use the hashtag #midkidstrike on their social media accounts on Middle Child's Day.
Hopman says while they're at it they should change their Facebook profile pictures for the day to show famous middle children like Lady Edith from "Downton Abbey," Bill Gates, Madonna or Malcolm from "Malcolm in the Middle."
"You know, like they do for other really important causes, like Supreme Court rulings," he said.
The comical IMCU bears the slogan "We are SO screwed!," and Hopman, 56, has gathered a small following of middle children who appreciate his efforts.
Author of the as yet unpublished "Smack Dab: A Middle Child's Story," Hopman also runs a Smack Dab blog, where he has several humorous greeting cards for the middle child you almost forgot on Middle Child's Day.
One features a child in a cape and aviator glasses with a shirt that reads, "Please notice me!" The e-card reads "Sorry we can't be with you on Middle Child's Day. We're having dinner with your brother and sister."
Hopman wasn't able to avoid his passing on his own plight, however. The father of three says he is keenly aware of the needs of his middle child, who is also his only daughter.
He also tweets @MidKidMusings where he has more than 4,500 followers. Fellow middle children appreciate his work building self-esteem among the oft-ignored group. "If we don't pick ourselves up," Hopman said, "who will?"