In one final attempt to safeguard the secret until the end, Sarah was spotted entering the bride's hotel at 3pm the day before the wedding in disguise.
Wearing an extravagant furry hood which hid her face, she dashed into The Goring Hotel to make sure the most highly anticipated outfit of the entire wedding was Princess-perfect.
Kate Middleton's decision is very telling, showing her wish to forge her own fashion path, rather than stick to the 'conservative' choices that critics have rather unkindly accused her of in the past.
Sara Buys, Camilla's daughter-in-law, was almost certainly influential in the process.
At her wedding to Tom Parker Bowles in 2005 the Harper's Bazaar fashion writer was resplendent in an Alexander McQueen dress fashioned by Sarah.
Her recommendation must certainly have filtered through to Kate, who has become closer to Camilla in recent months, lunching with her to discuss the wedding.
Speculation over the identity of Kate's couturier garnered extensive media coverage and racked up thousands of column inches as millions scrambled for any kind of far-fetched information or hints.
Which begs the question: just how did the bride manage to keep it all under wraps?
The odds were initially on her choosing veteran designer Bruce Oldfield, Diana's dresser, or London bridal couturier Phillipa Lepley.
But stories soon began to centre on the Alexander McQueen label when chief executive Jonathan Akeroyd reportedly let slip to a colleague that the house had won the commission.
But he quickly denied any involvement, saying: "Not at all. I am the CEO - I would know if we were doing it."
And Sarah's spokeswoman was also quick to issue a denial, saying: "Sarah is busy working on her new collection for the Tuesday show and she was as surprised as the rest of us to hear about this. She is not designing the dress."
Meanwhile, even Sarah's husband spoke out to quash the reports: "It would be amazing if she is doing it but I would think I would know about it and I don't."
But, the rumors refused to die, with seamstresses being photographed coming in and out of the headquarters.
To stop prying eyes, they apparently locked off areas of the atelier.
The staff also began arriving unusually early, but even they would have been in the dark about the final design, as they were reportedly only given specific sections to work on at any one time.
Amidst the frenzy, an insider told Grazia magazine: "Apart from when they are incredibly busy in the run-up to fashion week, everyone [usually] arrives at the same time.
"[But] there have been lots of whispering and secret meetings at head office recently."
At 11am on Friday, with two billion people watching, the results of their handiwork will be unveiled.