Ed Dunlop's dual Epsom and Irish Oaks winner will be tackling one mile and six furlongs for the first time at Doncaster following her excellent second behind the classy Midday in last month's Yorkshire Oaks.
With Ryan Moore, on board for Snow Fairy's two Oaks triumphs, contractually committed to Total Command, 'super-sub' Ahern is reunited with the horse for the first time since a winning ride in the Spring.
Snow Fairy is a 7-1 chance to make history and become the first filly since User Friendly in 1992 to win the stamina test.
And while George Duffield, User Friendly's victorious jockey, is among those to query Snow Fairy's stamina, Ahern is bullish – though he accepts the horse to beat is Dettori's mount Rewilding, the Epsom Derby third who became the warm St Leger favourite after winning York's Great Voltigeur Stakes emphatically last month.
"It's great to get the ride. The way she won at the Curragh on soft ground you would think she could stay all day and she won't mind the ground at Doncaster," said Ahern.
Yet Dunlop has said that he will not risk his heroine if the Doncaster ground becomes unduly testing overnight. He also accepts that Snow Fairy will be entering unchartered territory distance-wise.
"She has been a star to us. Whatever happens we won't forget what has happened already," said Dunlop.
"She surprised us on pedigree. We didn't think she would stay a mile and a half until we tried her over it and she proved she did.
"We are under no illusions that it will be very tough."
Top Irish trainer Aidan O'Brien seeks his fourth Leger through Midas Touch and Joshua Tree, second and third respectively to Rewilding at York.
The also-rans on Knavesmire on that day included the aforementioned Total Command but trainer Sir Michael Stoute says the 33-1 outsider should not be discounted.
"He had a skin infection and a joint infection and he was getting messed around.
"The ground was probably quick enough for him at York too. We felt he could be a Leger horse," said Stoute.
The most popular winner would be Dandino, trained at Gainsborough by James Given. He is owned by the Elite Racing Club's 12,500 members who each paid 200 for the privilige.
In many respects, syndicates are the way forward for ownership and Elite's owners will be out in force.
"Basically what we do is have a random ballot for the tickets. We let the winners know and they can tell us if they are available to go," said Elite spokesman Matthew Budden.
"There will be a few very happy people with tickets to the paddock and some who won't be happy, but they will still be there cheering him on. Having a runner in a big race or a Classic is what everyone dreams of. It's so exciting for everyone."
Yet, with Middleham-based Mark Johnston's Corsica expected to set the pace for Rewilding, local hopes primarily rest with Malton jockey Paul Hanagan. On the brink of becoming champion jockey for the first time, he rides outsider Theology.
Though the Jeremy Noseda-trained horse finished behind Arctic Cosmos, one of today's market leaders and who will wear blinkers for the first time, in the Gordon Stakes at Glorious Goodwood, the stable believes Hanagan's services will be an asset.
"We are not going to complain about having Paul aboard. The way he is riding at the moment, he could get a tune out of a rusty wheelbarrow," said Noseda's assistant, Tom Morley.
Yorkshire entrepreneur Sir Robert Ogden's Sans Frontieres carries English hopes in today's Irish St Leger.