5th May

On this day in 1777, Leonhard Euler was the first to use the symbol i for 1\sqrt{-1} in the paper De Formulis Differentialibus Angularibus maxime irrationalibus quas tamen per logarithmos et arcus circulares integrare licet [Of the most irrational Angular Differential Formulas, which, however, may be integrated by means of logarithms and circular arcs], presented to the St Petersburg Academy:-
Quoniam mihi quidem alia adhuc via non patet istud praestandi nisi per imaginaria procedendo, formulam 1\sqrt{-1} littera i in posterum designabo, ita ut sit i i = -1 ideoque 1/i = -i.
[Since there is still no other way clear to me of accomplishing this except by an imaginary procedure, I will denote the formula 1\sqrt{-1} by the letter i in the future, so that i i = -1 and therefore 1/i = -i.]