The end of the novel is complex: Woolf doesn’t just hand us the meaning on a silver platter (darn). We know that Septimus is dead, and that Clarissa’s party is coming to an end (it’s 3:00AM, for crying out loud!). But other than these facts, the message is not entirely clear.
For a novel so dark, personal, and, well, gloomy, the ending seems kind of uplifting. Well, it starts out with someone announcing Septimus' death in the middle of the party, but then things start to turn around. Clarissa soon sees the beauty of the sacrifice that Septimus has made with his suicide. Rather than feel pity, she’s heartened: Shakespeare's words "Fear no more the heat of the sun" (from Cymbeline), return to her. These words connect Clarissa and Septimus (he had remembered them earlier in the novel), and suggest an end to fearing death, something that has haunted Clarissa throughout the story.
Woolf ends by emphasizing the idea of Clarissa and Septimus as doubles:
"But what an extraordinary night! She felt somehow very like him – the young man who had killed himself. She felt glad that he had done it; thrown it away." (6.92)
Septimus' suicide has allowed Clarissa to see the beauty of life; his death means her rebirth. To emphasize this rebirth, Woolf has the woman across the way finally acknowledge Mrs Dalloway, "Oh, but how surprising! – in the room opposite the old lady stared straight at her!" (1.92). She has finally made a connection, albeit minor. After these great revelations, Clarissa returns to the party. Quite simply, she delights Peter with her return. He no longer denies the deepness of his feeling for her, and her presence changes the moment. A surprisingly optimistic ending indeed.