Sept. 1 is a big day for people who grew up in the magical world of "Harry Potter;" it's the day young witches and wizards board the Hogwarts Express to go to school. Some board the train as returning students while others are just starting their magical education.

For James Sirius Potter, Sept. 1, 2015, marks the day he started at Hogwarts; at least, that's what J. K. Rowling reminded millions of "Harry Potter" fans when she tweeted fans and asked anyone at King's Cross to wish James S. Potter good luck.

"He's starting at Hogwarts today," Rowling explained.

For those who need a little reminding, James Sirius Potter is the oldest son of Harry and Ginny Weasley. In Rowling's books, the couple has two other children, Albus Severus and Lily Luna.

The tweet was undoubtedly a pleasant surprise for fans, and one that immediately started a #BackToHogwarts Twitter trend that lasted for hours. Numerous Potter fans started tweeting photos of themselves on the famous Platform 9 3/4 at King's Cross Station while some sent their own quirky and imaginative replies.

A few hours later, the 50-year-old author (and Twitter) was soon inundated with so many tweets that Rowling worried the attention might be confusing or overwhelming for James.

But Harry Potter's eldest son safely reached Hogwarts, along with the rest of the students. They probably also had a good feast after being sorted and as expected, James was sorted into his parents' old House.

Potterheads know that there are four houses at Hogwarts School of Witchcraft and Wizardry – Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. Aside from Harry and Ginny, other famous Gryffindor alumni are former headmaster Albus Dumbledore, James' uncle Ron and aunt Hermione, Neville Longbottom, his namesake Sirius Black and Teddy's father, Remus Lupin. Lord Voldemort, when he was still known as Tom Riddle, was in Slytherin while the doomed Cedric Diggory was from Hufflepuff.

Rowling's surprise tweet has been retweeted more than 200,000 times already and is just another example of how the author connects with her books' very dedicated and active fan base. But there are fans who are concerned that tweaking the now finished story by adding little bits of information here and there might eventually ruin the magic of the books.

That danger might indeed be there, but for millions of Harry Potter fans, it's obvious that the tweets were just another way of celebrating a beloved book.