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.
"Party o' clock"