Which character celebrates a birthday on the day Myrtle dies?

On the same day that Myrtle is killed, Nick celebrates his thirtieth birthday. When Nick realizes that it is his birthday and tells Tom, he has a moment of clarity in which he becomes daunted by the prospect of entering another decade. In fact, he calls it a "portentous, menacing road of a new decade."
Although he does not yet know it, this day will become "portentous" and "menacing" for another reason. The death of Myrtle Wilson, which takes place shortly after this realization, sets in motion a chain of menacing events, which will affect Nick in a far more significant way than simply turning thirty. It will lead, for example, to Gatsby's murder and the end of his American Dream, alongside Nick's decision to leave the East and return home to the West.