Save the dates aren't just for weddings. Let people know about your event early so their calendars aren't full by the time you invite them.
I don’t know what it’s like where you live, but in Joburg a lot of small to medium geek conventions have something in common – the venue. There’s also some overlap in venues for the two big Gauteng boffer larps (MEAD Legends and Tales of Teana). You could do a lot of work to find a venue that’s uniquely yours, or you could follow the common sense approach.