Why was David's birthday memorable?

In chapter 9, "I Have a Memorable Birthday," David finds out that his mother is dead, and that his baby brother is probably going to die, too.
When he's called into the parlor, David thinks that he's going to get a gift for his birthday. Instead, he's told that his mother was seriously ill and died. He was very close to his mother and feels grief when he finds out that she's gone. He stays in the parlor for the rest of the day with Mrs. Creakle except for when she gives him time alone. He cries, sleeps, then cries more while arrangements are made for him to go home for the funeral. He finds out that his baby brother also died.
The death of his mother is a turning point in David's life. Her second husband doesn't care for David properly and sends him to work in a warehouse.

While David is at boarding school, his March birthday arrives. He is summoned to the school parlor. He happily anticipates a hamper of treats from Peggotty and hurries off to receive them.
What makes this birthday memorable, however, is not anything happy. He learns that his mother has died and that his infant brother is also expected to die, which he does. David is in a state of shock, and he sincerely mourns his mother's death, remembering her as the young, happy woman she was when he was small.
At home from Salem House, he learns that his beloved Peggotty has been given a month's notice, meaning she will be laid off in a month's time. Further, he is neglected by the Murdstones, and he realizes the Mr. Murdstone hates him more than ever. A mother's death is not the kind of birthday surprise a child hopes to have.