apart from the great tips that you've already gotten I would also recommend running Press Releases for 1-2 months before the festival starts, just to give enough time to these PRs to work their magic (spread the word far and wide). It will probably be expensive, yes, but festivals is extremely news-worthy content, so it will work great for you.
And come to think of it, if the event is local (as in, it's going to take place in the offline world) you could also run ads and TV / radio spots in your local media outlets. And you can take it 1 step further by running advertising campaigns in Google / FB ads (for local events social media promotion is a godsend)
EDIT: oh, I just noticed that you said "online festival"... Oh well, this leaves local advertising out of the question, but you can still run PRs and PPC advertising just as successfully