The passages that introduce the setting mention the end of the Great War in passing, only casually taking the time and effort to simply acknowledge that some have died. The importance was placed on the jovial setting that the end of the war had created for the better. Even when a car, a very new technological introduction to consumers at the time, backfires, people are surprised rather than afraid, even politely apologizing for it, despite the crowd that gathers having nothing to do with the incident. Even more telling is the fact that the sound of the backfiring was likened to that of a gunshot and yet very few people reacted in a negative manner, rather, they even entertained fantasies of having noticed that the prince of Wales, or the Prime Minister was in the car.
Conversely, Septimus Smith is shocked by the noise and falls back onto suffering. He doesn't share the bright optimism by the crowd, but a series if delusions. Unlike Mrs. Dalloway who dwelves into the past of her own will, Septimus Smith is perpetually haunted by his past, seeing his dead comrade among other things. I have not decided what the significance of this duality is, but the contrast that it shows reminds the readers that not all is well even at the war's conclusion.