One Day in December by Josie Silver

Laurie is on the bus home from a long day at work when she meets eyes with a man on the street. It is only for a few seconds, but it makes an impression on her. She spends the whole next year trying to find her mysterious bus boy, only to finally meet him when her best friend Sarah introduces him as her new boyfriend. From there, Laurie, Sarah, and Jack are woven into each other's lives.