January 8, 1981 wasn't meant to be the start of anything. Isabel Allende was living in exile in Venezuela when she got the call that her grandfather, back in Chile, was dying. She sat down to write him a letter, and that letter turned into her first novel.

The Direct Answer
She began The House of the Spirits on January 8, 1981, and it went so well she has started every book on that date since.
The date became a ritual for luck, and she's called January 8 a sacred day, per her official interviews.

The Letter That Became a Novel

In 1981 Allende was far from home. The 1973 coup in Chile had pushed her into exile, and she was working in Caracas when word reached her that her grandfather, then close to a hundred, was near death. She began writing him a letter, partly to hold on to the family stories he had told her over the years. He died before she finished. The letter kept growing, roughly five hundred pages of it, until it had become The House of the Spirits, published in 1982.

The book launched her career. So when it came time to write the next one, she reached for the same date and hoped the luck would hold.

Why She Kept the Date

Allende is open about the superstition. Starting on January 8 gives her a fixed line on the calendar, a morning she can't argue her way out of. She has described it as a sacred day. She comes to her office early, alone, lights candles for the spirits and the muses, and keeps fresh flowers nearby before she writes a word.

There's a practical side too. A set start date removes the endless question of when to begin. The next book always opens on the same morning, so the decision is already made.

On January 8, 1981, I was living in Venezuela and I received a phone call that my beloved grandfather was dying. I began a letter for him that later became my first novel.

Isabel Allende, official website interview

For the wider picture, here's the full Isabel Allende writing routine.

What You Can Steal