I just noticed that tomorrow (March 13, 2015) will be "Friday the 13th". I was curious to know when the next/previous occurrence of this will be and figured it would make for a great Instant Answer.
We already have an API Key which should be usable for development. I'm not sure how easy it is to get just this information from their API though -- it seems like you might have to get a list of all holidays from a given country which isn't ideal.
A "holiday search" Instant Answer powered by Time&Date is coming soon, but I think this one is worthy of being a separate IA.
Alternatively, a Perl library could be used to calculate the dates, and this could be implemented as a Goodie.