Kate Middleton turns 41 on Monday — the day before her brother-in-law Prince Harry’s heavily publicized memoir “Spare” finally goes on sale.

It will likely not be the happiest of days for the princess.

“Harry and Meghan do always have a knack of picking their timing,” said one highly placed royal source who knows Prince William and Kate. The Prince and Princess of Wales, the source said, have been left “reeling” from Harry’s bombshell book.

But it wasn’t always like this. In fact, when William popped the question to Kate after eight years together, his younger brother Harry was thrilled, saying in 2010 about his brother’s wedding: “It means I get a sister, which I have always wanted.”

The woman whom Harry once spoke about in glowing terms is presented as cold and uncaring in his book, however.

The publication of Prince Harry’s memoir, “Spare,” has laid out the surprising complications in Kate Middleton and Meghan Markle’s relationship.

“Kate, William and Harry were such best buddies, they were inseparable,” said the Wales source. “These are the words of a man so literally blinded by love he can’t see straight.”

But in “Spare,” Harry blames Kate for taking his beloved brother “Willy” away from him, writing that after their April 2011 wedding: “The brother I’d escorted into Westminster Abbey that morning was gone — forever. Who could deny it?”

Contradictorily, Harry writes how he’d long dreamed of forging a working partnership with William, Kate and his own wife — even before he met and fell in love with the “Suits” star in 2016.

Before Meghan Markle came into the picture, Prince Harry and Kate Middleton were regularly seen palling around at royal events.

“I also said I couldn’t wait for them to meet her, that I looked forward to the four of us spending lots of time together, and I confessed, for the umpteenth time, that this had long been my dream — to join them with an equal partner. To become a foursome,” Harry writes.

Instead, Kate and Markle were never close friends.

From the minute her relationship with the prince was revealed, Markle was publicly compared to Kate, Britain’s future queen: the English Rose against the outspoken American actress.

In “Spare,” Harry suggests that his bond with William was changed by his brother’s marriage to Middleton.Getty Images

Harry writes about Markle crying over a perceived slight from Kate.Getty Images

According to Harry’s book, as attention shifted to Markle, Kate felt that she was being “forced to compete with Meg.” He details how Kate “grimaced” when Markle asked to borrow some lip gloss at their first “Fab Four” event, a Royal Foundation Forum appearance in February 2018.

The Duke of Sus𝓈ℯ𝓍 writes that the outing marked the beginning of the women being divided by the press after journalists “sensed” tension during the panel.

“I think it had Kate on edge while putting her and everyone else on notice that she was now going to be compared to, and forced to compete with, Meg,” Harry writes.