Tomorrow marks the anniversary of Robert Louis Stevenson's death but the real event of the year for his fans was his birthday celebrations. November 13th was the 150th birthday of the Scottish author who wrote Treasure Island and The Strange Case of Dr Jekyl and Mr Hyde, among other works.

Academics and critics from Edinburgh to Monterey and Samoa celebrated Stevenson's life and work. But celebrations of a different kind were also held at Oranmore, Co Galway.

Heather Finn and her family marked her birthday on November 13th by popping open a bottle of champagne. But 20-year-old Heather doesn't just share Stevenson's birth date.

While Heather was born on February 4th, Stevenson's birthday was "left" to her by her grandmother Anita Leslie-King, who received it from her aunt Annie Ide.

Annie was 12 years old when she moved to the island of Samoa in the South Pacific in 1891. She went with her sisters and her father, Henry Clay Ide, who was US Land Commissioner at the time.

Robert Louis Stevenson, his wife, Fanny, and her children from her first marriage had settled in Samoa to escape the Scottish climate which caused the author's illness throughout his life. The Stevensons took to the Ides and opened their home, known as Vailima, to them.

But Annie had the misfortune to have been born on December 25th and so had to share her birthday with Christmas Day and all the fuss that went with it. As a child she thought it was a miserable fate to have to do without presents and all the attention a birthday brings.

When her father mentioned Annie's complaint to Stevenson one day the author decided to "give" his birthday to her.

On June 19th, 1891, he sent Henry Ide a copy of a document transferring his birthday to Annie. "I, Robert Louis Stevenson . . . have transferred and do hereby transfer to the said A. H. Ide, all and whole my rights and privileges in the thirteenth day of November, formerly my birthday, now, hereby and henceforth, the birthday of the said A. H. Ide," the document read.

He said the transfer would allow her a "proper birthday" and the "eating of rich meats and receipt of gifts".

"And I charge her to use my said birthday with moderation and humanity . . . the said birthday not being so young as it once was and having carried me in a very satisfactory manner since I can remember," the document continued.

But the legal script warned that "in the case the said A. H. Ide shall neglect or contravene either of the above conditions, I hereby revoke the donation and transfer my rights in the said birthday to the president of the United States of America for the time being".

Annie also became Stevenson's "named daughter" and she took the name Louisa, which she used for the rest of her life. He wrote her letters and sent poems he had written for her.

She celebrated the birthday throughout her life, even after she moved back to the US with her family. She married a Congressman, William Bourke Cockran, and lived in Washington DC but had no children. She passed the birthday on to her niece Anita Leslie-King "so that a mortal date can be carried on immortally".

Anita's mother, Marjorie, married Shane Leslie of Castle Leslie, Co Monaghan. Anita celebrated November 13th with a few drinks and chat about Stevenson and her ancestors' time in Samoa with him.

Although Heather Finn was six years old when her grandmother died she began celebrating November 13th only a few years ago. She says the birthday skipped her own mother possibly because she was very close to her grandmother and was left a lot of her things.

"We were out for dinner one night when I was told about the birthday and I didn't really understand what it meant," says Heather, who studies fashion and art history at the National College of Art and Design in Dublin. "I still don't understand what it means to have someone's birthday."

Like many children Heather read Stevenson's books but was unaware of the special link between her and the author.

"I would know Robert Louis Stevenson from Treasure Island and Kidnapped and I loved the books and the films. But I had no idea when reading the books that I had his birthday. Nobody tells me anything."

She says she can understand that it must have been frustrating for a 12-year-old's birthday to fall on Christmas Day. "I could imagine how awful it would be to have your birthday at Christmas. Everyone would hate it. It's such a special day and you would have really missed out."

She herself also celebrates her real birthday on February 4th. But next year she turns 21 and intends organising a proper bash on November 13th.

"Next year I'm definitely going to make a big deal out of it. And I want presents just like Annie managed to get," she says.

Heather says she will also pass on the birthday if she has a daughter. She says she intends to keep alive the special tradition in her family of celebrating Robert Louis Stevenson's birthday as their own.