Tết happens twice in the story: once near the very beginning, when Hà very sneakily touches her toe to the tile floor of the house to bless the house first, and once at the end of the story, when--knowing what her daughter had done the year before--Hà's mom gets Quang to bless the house this time before Hà can have a chance to repeat her little act of defiance.
So how is this blessing symbolic?
"Only male feet can bring good luck," Hà explains to us near the beginning of the story. By allowing a male in the family to be the first in the new year to touch his bare feet to the floor of the home, Hà's family believes that luck will come to them throughout that year. In other words, the action of male feet touching the floor of the home at the start of the new year is a symbol of blessing and luck.
Being the only sister in the family, though, Hà hates this. The unfairness of being a girl and not having the special touch to bless a house rises up as "an old, angry knot" in Hà's throat. Flash forward toward the end of this book, and Hà shouts, "I hate being told I can't do something because I'm a girl!" For this reason, we can understand her act of defiance, which is also important to the plot of the story as the turbulent year makes Hà wonder if her toe touching the floor brought on all the troubles that year.
But her confession about touching her toe to the floor the previous Tết prompts her mother to admit the limitations of the symbolism of that tradition: "I was superstitious, that's all. If anything, you gave us luck because we got out and we're here."