Fra Angelico's "Annunciation"St. Therese especially loved the mystery of the Annunciation, and celebrated it every year. At the first inquiry into Therese's sanctity in 1910, her sister Celine testified:  

She had a particular devotion for the mystery of the Incarnation, which she would observe devotedly every 25th March. She loved to contemplate Jesus in his childhood. She once said, “I should like to die on 25th March, because it was on that day that Jesus was the smallest.1

In 1888, the feast of the Annunciation, transferred because of Lent, was celebrated on April 9, the day Therese entered the Lisieux Carmel.  God must have understood that her "yes" to her Carmelite vocation would be a profound echo of Mary's "yes."  

Therese's understanding of what happened when the angel Gabriel came to Mary is remarkably realistic and down-to-earth.  In July 1915, her sister Pauline, Mother Agnes of Jesus, testified at the second process:

She was very simple and had little experience of evil. Fearful of discovering it, as she acknowledges in her autobiography, she entrusted the protection of her purity to the Blessed Virgin and Saint Joseph.  Later on, she came to understand that everything is pure for the pure of heart. Seeing that she knew about the realities of life, I asked her who had enlightened her. She said that she had discovered them without even looking, from observing nature, the flowers and birds. She added, “The Blessed Virgin knew all these things. For she said to the angel, on the day of the Annunciation, “How will this be, since I know not a man?” Knowing things is not evil. All that God has made is very good and very noble. Marriage is a beautiful state for those whom God has called to it; it is sin which distorts and soils it.”2

  Therese emphasized that Mary, who was purity itself, knew the facts of life when the angel came to her, and that there was nothing wrong with her having that knowledge; Therese, the wise and innocent child of God's mercy, understands with Mary that God has made nothing that is not very good.  

1http://archives-carmel-lisieux.fr/english/carmel/index.php/4-genevieve-de-sainte-therese-ocd

2http://www.archives-carmel-lisieux.fr/english/carmel/index.php/6-agnes-de-jesus