The Shadow attack on Babylon 5 took place in December, 2260, when the Shadows mounted an open attack on the station. This occurred while John Sheridan was off-station, being taken to Z'ha'dum by Anna Sheridan. While he was gone, the Shadows intended to remove his support mechanism: the station itself. They called off their attack upon receiving word of the destruction taking place on their homeworld.
As the Shadows pulled out, they captured Michael Garibaldi as a pawn for the next phase of the war.
There were several instances of members of Babylon 5's crew witnessing an alternate timeline where the station is destroyed by the Shadows in this attack.
Michael Garibaldi went out to Sector 14 to evaluate a message seeming to come from B5 a few weeks in the future. In this message, he sees Susan Ivanova issuing a general mayday to all ships in Grid Epsilon, begging for help. In what appears to be a time loop, he watches the station get destroyed over and over again by a massive armada of Shadow vessels.
Jeffrey Sinclair, while aboard Babylon 4, witnessed an alternate timeline where he refused to go back in time to the First Shadow War. As a result, the Shadows were far more powerful and had three times as many ships as they would have otherwise. He was present at the evacuation of Babylon 5, an event that resulted in the deaths of John Sheridan, Susan Ivanova, and Michael Garibaldi - among many, many others - before finally witnessing the destruction of the station itself.