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1.
n. That time of night when teenagers and young adults can be expected to arrive at a party.

The expression is used mostly among college-aged adults who typically party-hop or bar-hop on the weekends. The ambiguity of the phrase avoids the potentially embarrassing situation of a guest arriving too early for a party or a host(ess) waiting for guests to arrive. See also party foul.

The term is most often encountered on Internet invitations or mass e-mails advertising a private party.

The time is generally understood to be between 10pm and midnight.
"What time are you heading over to D's house?"

"Party o' clock"
by mxm February 08, 2009