This date is uncertain by a single day at most. Caryll uses Lady Mary
to defend Hortensia on Saturday May 6 ["Your Dulcinea … has ne'er a
glance for you o' Saturday!", Chapter IX], his exposure of Rotherby at
White's was "three or four days later" [Chapter IX], and the duel was at
"seven o'clock next morning" [Chapter XI]. I've chosen May 10 over May
11 solely because the former is my birthday.