His story, shared to Reddit's infamous AITA ("Am I the A--hole") forum, details the bizarre story behind why his mother has treated him differently from his four other siblings since birth, behavior which he says ultimately caused his parents to split up.

The whole story led to some pretty brutal comments towards the OP's (a.k.a. the "original poster") mother ... and a lot questions about one detail in particular.

Read on to see the full story and how Redditors reacted.

The man kicked off the story by sharing his family's dynamic.

"I'm (21m) the middle child of five siblings. And I always knew my mom treated me very differently from the rest of my siblings. It caused my parents marriage to end because my dad made it very clear he wouldn't tolerate her being so different with me," the man wrote.

"This was after he tried to figure out why she was so different with me. A few close family knew and said it was [Postpartum depression] from when she had me, that apparently only affected her ability to bond with me and not any of my four siblings, just me, the middle kid. I have a sibling one year younger than me and she bonded with her just fine," he continued.

He then revealed how he discovered the reason behind his mom's "lifelong" beef.

"I was a teenager when I got into a fight with mom one day and she accused me of always hating her and never wanting her to be happy. I asked what the f--k she meant by that and she told me that I had to come on the one day she had other plans, that it was bad enough I was overdue when I was born, but I prevented her from reading a book that released that day that she'd made plans to read months in advance," he wrote in his post.

"She told me I just had to be born that day so she was too tired to read it on the release day and she was weeks behind. She said I couldn't even be born normally like my siblings and I was a c-section baby. She told me it f--ked her plans so bad and I never seemed to care," he continued, before adding that she was also furious that he cried more than his other sibling.

"She said I started f--king her over the day I was born," he said, adding that his mother said he "made her seem like such a monster and none of my siblings were like that. She told me I clearly always wanted to come between her and dad. She was pretty serious about it too."

According to OP, he wondered whether that was the true source of her ire -- claiming he and his father then took a DNA test just to see if she was "making up a dumb reason so we won't know she cheated." The test, he said, confirmed he's "definitely dad's kid."

To make matters even worse, OP said he gets a "hard time" from family friends who assume he's "s--tty" to his mother for "no reason." He was pushed over the edge at a recent family gathering, however, and finally told the truth.

"They put the bad relationship on me and at my grandparents house on Saturday I got so tired of the comments and mom sneering at me the whole time, they I exposed what she said to me when I was a teenager and the blame she put on me being born the wrong day for our bad relationship," he revealed. "Mom half admitted it because she got so angry she verbally lashed out at me. It created a s--tshow and some family members think I made a big deal out of nothing but others were disgusted with mom."

"I was told I was wrong to expose her like that. Mom told me I had no business doing that," he concluded, before asking whether he was the a-hole in the situation.

With over 1.2K comments at the time of writing, it's clear the internet does not believe OP is in the wrong, with Reddit giving its official "Not An A--hole" stamp on OP's post.

In the most upvoted comment, one Redditor blatantly told OP his mother "hated" him: "NTA. She hated you, her own child for something you couldn't control. That is bizzar (sic) and absolutly (sic) wrong from her. I'm sorry, that you have someone like that as a mother and I hope you have at least a parent figure in your dad. Do not give 5 cents about anything your mother says anymore. She doesn't deserve that you care about her."

While another noted how lucky OP is to have such a supportive father, "I'm so glad you have your dad. Look, you're not obligated to have a relationship with a rotten person. 'Mom' is a title earned, and she crumpled it up and threw it out the day you came into this world. If you like individual family members, it is possible to have a relationship with them independently from your maternal genetic donor. Life is too precious to let her poison it any further. NTA."

"My dad is great," OP wrote in some of the replies. "He did his best to make sure I didn't suffer as much from mom's attitude toward her. His only regret is he couldn't keep me from her entirely but that was never going to happen and I know that."

"Born on the wrong day? That's a new level of ridiculousness. You did the right thing exposing her," wrote another commenter -- sparking a response from OP reading, "Yep and all because of a book too which only adds to the insanity of this."

On a lighter note, many Redditors were curious to find out what book his mother wanted to read so badly and tried to work it out based on OP's age. Many assumed "the book was about teen wizards," suggesting Harry Potter and the Order of the Phoenix -- which came out June 2003. However, OP quickly shut that down.

"It wasn't Harry Potter because that book came out after I was born. I'm not actually sure which one it was though. She didn't tell me during her long rant about it, surprisingly," OP revealed.

With Harry Potter off the table, another made a list of other possibilities. "OP, I'm so curious, and I have a number of possible theories of books that sold really well and were also the kinds of books that might have been highly anticipated months in advance.

The list included:

1. The Second Summer of the Sisterhood (April 22), sequel to Sisterhood of the Traveling Pants
2. The Da Vinci Code (March 18)
3. Club Dead (April 29) Sookie Stackhouse/True Blood book #3
4. Two Nicholas Sparks books, The Wedding (September 1) and The Guardian (January 1)
5. Crossroads of Twilight (January 7)
6. The Queen's Fool (February 4)
7. Devil Wears Prada (February 6)
8. Oryx and Crake (April 22)
9. To Sir Phillip with Love (July 1), Bridgerton series
10. Janet Evanovich, To the Nines (July 15)

What do you think?