Bob Minzesheimer

USA TODAY

As John Green's novel about teens with cancer, The Fault in Our Stars, returns to No. 1 on USA TODAY's Best-Selling Books list – its 14th week atop the list -- Green is preparing to celebrate Esther Day this Sunday.

Esther Earl, who helped inspire Green's 2012 novel, was an avid reader and creator of quirky YouTube videos. She had just turned 16 when she died of thyroid cancer in 2010.

Green says that toward the end of Esther's life, he and his brother Hank were talking to her and suggested a holiday in her honor, to be celebrated on her birthday, Aug. 3.

"She came up with the idea of celebrating friends and family and love," Green says in an e-mail to USA TODAY. "The kinds of love that are too often overlooked in our culture: love among friends and family."

Green adds that Esther Day "is about telling the people you love that you love them, even when it's difficult to do so -- in fact, especially when it's difficult to do so. We have a holiday for romantic love, but love isn't just about romance, so that's why Esther created Esther Day."

Each year, John and Hank Green, known to their video fans as the VlogBrothers, post an Esther Day video on YouTube. Last year's attracted 2.5 million views.

This Sunday, Green, along with Esther's parents, Lori and Wayne Earl, will be in Orlando, Fla., at LeakyCon, an annual convention of Harry Potter fans. (Green first met Esther at a LeakyCon in Boston in 2009).

By tradition, Green says, it's "the only day of the year in which Hank and I say 'I love you' to each other -- before the first Esther Day, we hadn't said it in years -- and it's become a hugely important holiday…We spread the message of Esther Day across social networks, and it's great to follow the #estherday hashtag and see all the love being expressed around the world."

MORE: How Esther Earl became an author after her death

Esther's parents compiled her writing and essays about her in a book published in January,This Star Won't Go Out. Green wrote the introduction.