According to some, St. Agnes appeared to her parents eight days after her martyrdom.   The ancient sacramentaries show clearly that this day celebrates the actual earthly birthday of St. Agnes; the nativity of John the Baptist and of the Blessed Virgin are thus celebrated because they were free from original sin; the earthly birth of St. Agnes is celebrated because her whole life abounded in divine grace.
(Source: Fr. Lasance, The New Roman Missal)