Sal is casting her mind back to that sad, fateful day when her mother walked out on her. In retrospect, it seems obvious that something of this nature was about to happen.
On the day before she left, Sal's mother asked her daughter several times if she wanted to take a walk with her in the fields. Sal didn't want to go and got quite angry with her mom when she kept asking if he wanted to go.
Although she didn't know it at the time, it seems that Sal's mother just wanted to spend as much time as possible alone with her daughter before she took off.
In any case, Sal's mother left the very next day without saying goodbye to anyone. However, she did leave Sal a letter, in which she explained that if she'd said goodbye it would have been too painful and would have sounded too permanent. She went on to say that she would be thinking of Sal every minute and would return before the tulips bloomed.
But as Sal ruefully reflects, her mom did not keep her promise; she did not return before the tulips bloomed. And Sal has been living with the impact of her mother's sudden departure ever since.