Yours is a question marketers consider all the time: change product, tweak conversion rates, or just the number of potential people to convert.
First of all, congratulations on having a popular draw to your community that brings participants who might not normally even know your community exists. You just increased your overall awareness which is a wonderful thing.
I'm presuming changing the scope of your community topics to something closer to your event the rest of the year is not an option (changing the product so to speak). That would reduce the "niftiness" you were referring to.
First, just measure how good your retention is. How big was the spike of new people who joined your community? What is their engagement rate? How long do they stay?
Perhaps increasing engagement or retention rate of all these new people is not the right approach. For example, maybe historically 10% of your target audience really will find your normal topics interesting and make good community members.
Then a better approach is to just magnify the overall spike of people who come to your community during this fun event, since you already know this has mass appeal. Advertise on other relevant sites. Try to get relevant bloggers to post about your event. Syndicate the best questions and clever answers to other places to reach more target audience.
If only 10% of your audience is historically right for your community, but you reach 3X more target audience during this time, that's still a 3X increase on new long term members.