This answer is arranged in the sequential order of the novel.
(a) The narrator of F. Scott Fitzgerald's The Great Gatsby, Nick Carraway, is the character who nearly forgets his 30th birthday, but suddenly recalls it at the hotel in New York.
(b) It is Michaelis, a young Greek man who runs a coffee shop next door to George Wilson's garage, who comforts Wilson and tries to help him with funeral arrangements.
Elaboration on answers:
(a) In Chapter Seven, Nick, Gatsby, Jordan Baker, Tom Buchanan and Daisy (who wonders "What'll we do with ourselves this afternoon? And the day after that, and the next thirty years?") decide to drive to New York City and try to cool off on an oppressively hot day. Not only is the day hot, but there is much tension in the air as Tom boasts of what he owns and Daisy's voice is "full of money." Gatsby decides to "play his hand" and he informs Tom that his wife does not love him. But Tom trumps Gatsby and conquers Daisy again:
Her frightened eyes told that whatever intentions, whatever courage she had had, were definitely gone.
In the midst of the heat and emotional tension and its aftermath, Tom passes the whisky and asks Nick if he wants any. When Nick does not respond, Tom asks again.
"No... I just remembered that today's my birthday."
I was thirty. Before me stretched the portentous menacing road of a new decade.
Nick realizes the frivolousness, selfishness, and amorality of those with whom he is seated.
(b) In Chapter Eight, it is interesting that the neighbor to George Wilson possesses the name Michaelis; like the archangel whose name he bears, the humble Michaelis assumes the role of messenger who informs the police that George Wilson has been "acting sort of crazy" on the next morning when Wilson has suddenly disappeared.
Perhaps, too, his name connects with allusions Fitzgerald has made with Gatsby as a myth-like character (the Roman Trimalchio in Chapter 7), signifying that the lives of these Americans have been false and artificial in their pursuit of their materialistic goals.