On <answer>July 25, 2015</answer>, the 18th and 19th satellites were successfully launched from the Xichang Satellite Launch Center, marking the first time for China to launch two satellites at once on top of a Long March 3B/Expedition-1 carrier rocket. The Expedition-1 is an independent upper stage capable of delivering one or more spacecraft into different orbits.
When were the 18th and 19th satellites for the BeiDou system launched?
July 25, 2015