From the official release announcement:
The Curse of the Sad Surprise
To everyone in Valoran,
so he wouldn’t be ignored,
Amumu sent an invitation--
an Urf Day party right next door.
So glad he’s born,
Of spatula-slapping fame.
Amumu wrote of the birth of Urf,
and asked if they’d all play a game.
At last, the morning of the party,
Urf Day was finally here.
Out the window, Amumu looked,
but not a single friend appeared.
So young Amumu took tiny steps,
and traveled through the land.
But even when Amumu stood
and peeked into their home,
He saw they partied without him,
And he was still alone...
Attend the funniest sad party in all the land and cheer up Surprise Party Amumu (1350 RP*) with Definitely Not Udyr (975 RP,) Archduke Nasus (750 RP,) Order of the Banana Soraka (750 RP,) or Urf the Nami-tee (750 RP!)
*On sale for 975 RP through 23:59 PDT on April 6th"
Now let's party on for a better look at each of the new skins:
[While his W is active, the rain puts out his candle :( ]
[Sad Face = Amumu missed his ult. Happy Face = Amumu hit an enemy with his ult!]