It appears that Pushkin actually started writing Eugene Onegin on May 9th, rather than May 28th, as I previously wrote here. Here's Nabokov :
"I have studied a reproduction of Pushkin's draft of the first stanza, and (as was conclusively shown in 1910 by P. Shchyogolev) the initial date should be taken to be "May 9" not "May 28 [as written below it, with both words underscored] at night". Furthermore, Pushkin himself in his recapitulation (Sept. 26, 1830) noted that he had begun EO on May 9, 1823." (- from Nabokov's intro to his 4-vol. annotated translation)
Curious about the underscored note, though.