Effective conference promotion combines three channels. First, your own database: an email to existing contacts converts better than almost any other channel. Second, LinkedIn with regular content about the theme, interviews with speakers and programme previews. Third, partners: organisations that share your conference with their network reach an audience you don't have yourself.
Announce early, as soon as date, venue and at least one keynote speaker are confirmed. Don't wait for the full programme. Communicate consistently: a series of posts across several months that reveal the programme step by step. That consistency keeps potential attendees engaged and increases the chance they actually show up.