A complete event communication plan contains at minimum: the event goal and communication goal, an audience analysis per segment and the core messages per audience. In addition, a channel overview with rationale, a communication timeline with all planned communications, a division of tasks and KPIs for evaluation afterwards.
Optionally add: a crisis plan, a social media guide for staff and a content plan for the aftercare phase. The bigger the event, the more extensive the plan. But even for smaller events it pays to put at least the timeline and core messages on paper.