"Yelda" is the term given in Afghanistan to describe the longest night of the year, the first night of winter. As such, Amir remembers staying up all night with Hassan, as was the tradition and listening to fantastical stories. However, as an adult, he discovers that yelda has another significance: yelda is the endless night filled with "tormented lovers" who wait up for the night to end and for the sun to bring their loved one to them. Thus yelda becomes an important symbol to describe the obsession and deep love that Amir feels towards Soraya. Note what he says about how yelda fits into his desires:
After I met Soraya Taheri, every night of the week became a yelda for me. And when Sunday mornings came, I rose from Bed, Soraya Taheri's brown-eyes face already in my head... The morning sun to my yelda.
Thus this Afghan festival or important date is used as a symbol of the author's deep feelings towards Soraya and his attraction and love for her.