-I don't know if we have any of the Guild Crew working on event coverage, but what's the harm of posting it twice? Exactly!-

When?:
Japan: November 27th 2009 to January 11th 2010 (OVER)
America and Europe (PAL): July 31st 2010 to August 27th 2010

Where?:
Over Nintendo Wi-Fi Connection, be it a Hotspot (McDonald's) Your house, ect.

What does it do?: This Event in Pokémon Heart Gold & Soul Silver gives you the Enigma Stone (Key Item). Take this item to the Pewter City Museum and take it to the guy who revives your fossils. He will discover that it is the Soul Dew. Steven Stone will tell you what Soul Dew is (Don't want to spoil it). Once you leave, Latios or Latias will arrive and challenge you depending on the version you have. Latias in Soul Silver, Latios in Heart Gold. This event is currently available only over Wi-Fi, so get those Routers booted up starting July 31st!