"Serenity now" is a phrase that is supposed to be uttered to calm you down if you are experiencing a high level of stress, anger, or anxiousness. If you are extremely angry at somebody and are about to impart, say, physical violence on that somebody, you close your eyes, take a deep breath, and say "serenity now" as many times as it takes until you are calmed down.
However, in the television sitcom Seinfeld, where the concept is introduced, it is actually, to much comedic delight, used more as an exclamation. During fits of rage during the episode it is yelled very loudly and does not actually do any calming down.
To fans of the show, and perhaps by now to many more, this has become a popular statement to yell during periods of intense frustration or anger to either a)express how angry you have become or b)try to add some levity to the situation by referencing a hilarious tv show. Whichever is the case, it has entered our pop culture lexicon (maybe only in America, but perhaps elsewhere) and has become a staple of rabid Seinfeld fans.
The phrase first appeared in the eighth (and final) season episode #159 entitled "The Serenity Now" which aired October 9, 1997. Par for the course for Seinfeld, even though virtually every character in the episode is involved in a different subplot, all have something to do with the "serenity now!" plot device. The main plot and genesis for the phrase is when Frank, George Castanza's father, has to utter the phrase every time his blood pressure goes up. Jerry Stiller plays Frank Castanza, and it is him that most people remember exclaiming "serenity now!" in his oft-wheezing, breathless, gasping anger-filled way, something that is sort of a trademark for the curmudgeon old actor. (He plays basically the same hilarious character in King of Queens.)
In the episode, Frank starts a business selling computers from his garage for which he employs George (Jason Alexander) and a man named Lloyd Braun (Bruce Mahler) who is seen by George as his arch rival. The business causes Frank a lot of stress, particularly when his wife Estelle (Estelle Harris) constantly complains about it being in the garage (by the end of the show she ends up driving a car right into it!), which fuels the need to say "serenity now." George, who helps Kramer (Michael Richards} install a screen door for his apartment, cannot keep up with Lloyd so he pretends to constantly be on the phone selling to nobody to impress Frank. But he must do something with all the computers he is supposedly selling so he has Kramer keep them in his apartment. Meanwhile Kramer is harassed by untamed youths in the apartment building as he sits outside his screen door enjoying the great outdoors. In one of the funniest scenes of the series, towards the end of the episode, Kramer can stand it no longer and runs to his apartment and, listening to it from Jerry's apartment, you can hear him smashing the computers up in between very loud exclamations of "SERENITY NOW!". Kramer, who had touted the phrase's usefulness throughout the episode, comes back to a surprised Jerry and despondent George and hails how well the phrase worked.
Steve Koren, the writer of the episode, mentioned in Writers magazine that the "serenity now" is based on a true story.