Cult of Mac reported that if you reword the question to when the gates of hell are opening, Siri will say they already opened on May 14, 2005.
As the website notes, the odd apocalyptic responses are glitches in Apple’s Wolfram Alpha, which is the engine that drives Siri.
According to an Apple forum discussion, “Opening Gates of Hades” might be in reference to “Chinese Ghost Month.” But others speculated it has to do with the Muslim holy month of Ramadan, where the gates of “heaven open and the gates of hell close.”