party; surprise party
“Jean avait organisé chez lui une surprise-partie. Nous devions tous nous retrouver à trois heures. Nous étions une quarantaine de garçons et de filles qui dansions gauchement, joyeusement.” — Jean had organized a party at his place. We all had to get there at three o'clock. There were about forty of us, guys and girls, dancing awkwardly but happily.
From English surprise party; sociological note: parties were social events where invitations were sent out to the invitees, be they children or teenagers. The novelty of the surprise-partie was its informal organization. When this became the norm, the word was re-analyzed to mean "a party thrown to surprise the one in whose honor it is made", and, in its original sense, shortened as surpat', surpatte, surboum.