Invite any breathing journalist to attend. Don't exclude anyone, even those the CEO hates. There is nothing more frightening for a PR person than walking into an empty press conference. Usually, the Daybook is posted on the wire the night before and morning of an event to which the press is invited. Make sure your event is listed.
Journalists are busy and make schedules, just like everybody else.
So get to them early enough with the announcement of your conference. That includes contacting them the morning of the event. Deadlines for both print and broadcast loom larger as the day proceeds. Therefore, a 1 p.m. press conference is pushing it. And anything after that, unless it's the announcement of World War III, will not be attended.