The day of the attack Excalibur was participating in a series of war games meant to test the training of the crew of the Argus, a Mercury class battlestar(same class as the Pegasus) and the newest in the fleet. The Excalibur was playing the part of a pirate force and as such was using older Mark II Vipers as it's primary fighter compliment. The Argus was tasked with tracking it down and eliminating it. In fact both battlestars were in the middle of simulated combat when they received word of the attack. They were then given orders to rendezvous with the rest of the fleet under Admiral Nagala at a point near Picon in preparation for a counter attack on the Cylon forces. The Excalibur jumped to the rally point and along with several other battlestars moved in to attack the Cylon basestars that were bombarding Picon. Needless to say the battle didn't go well for the colonials with the majority of the colonial ships mysteriously shutting down or suffering malfunctions just before the battle began. Excalibur was the only battlestar not to be effected the malfunctions not only left the fleet vulnerable but also left the large number of civilian ships that had been under the fleet's protection open to attack. With no other options Excalibur issued orders for the civilians to jump away to a new rally point, covered their escape, recalled any/all Vipers and Raptors who could still respond, and jumped after them. From that the course of events followed the timeline that's been laid out.
If anyone has any further questions please feel free to ask.