7 November 1865 Reference:
The date is mentioned at the Illustrated London News of Saturday 11 November 1865:
On Tuesday the Pneumatic Railway between Holborn-hill and Euston-square - a portion of the tube which is to connect the General Post Office with the terminus of the London and North-Western Railway - was formally opened. The distance is over two miles, and the time required for the transmission of parcels is about seven minutes. Many distinguished persons were present, and some of them were blown through the tube.