Question: What does the Gospel mean when it refers to Mary as the "espoused wife" of Joseph? (CSG)
Answer: An "espousal" or "betrothal" is a formal agreement to marry. In modern American terms it would be something more than an engagement, yet less than a marriage.
In the Jewish culture of New Testament times, a young woman became marriageable at age twelve and a half. At a ceremony of betrothal, the bride and groom would exchange marital consent, but normally the bride would remain in the house of her father for somewhere between three months and a year. Marriage had the aspect of a man acquiring title to his bride, and only later did he acquire actual possession.
We know that Mary and Joseph had completed the contract of betrothal from the testimony given by St. Matthew (i: 18) and St. Luke (ii: 5). What causes some confusion, though is Luke's reference to "Mary his espoused wife" when Mary and Joseph were already in Bethlehem, seeking a place to give birth to our Infant Lord. For them to be in Jerusalem together, Joseph had obviously taken Mary from her father's house, and it seems clear that their betrothal had already been converted into a marriage. As he was aware of the circumstances of our Lord's conception by the Holy Ghost, Luke was probably following the custom of referring to a non-consummated marriage as an "espousal."
The Scriptures don't record exactly when Mary and Joseph were married, but the first chapter of St. Matthew's Gospel describes the circumstances. Verse 18 speaks of them being "betrothed ... before they came together." Joseph, on learning that she is pregnant with the child Jesus considers what he ought to do in verses 19-20, the situation is explained to him by an angel in verses 20-23, and in verse 24 we are told that he "did as the angel commanded him, and took unto him his wife." Presumably, this took place in Nazareth, as he would not have brought her to Bethlehem if she was still a member of her father's household. It (obviously) has to have followed the Annunciation (Luke i: 26-38), and very likely followed the three months Mary spent with Elizabeth. No mention is made of Joseph in the narrative of the Visitation (Luke i: 39-56), which has Mary leaving Nazareth "in haste into the hill country, to a town of Judea (Ain Karim)," and concludes about three months later with Mary "returned to her own house." That would put the events of Matthew i: 19-24 at roughly the fourth month of Mary's pregnancy; a reasonable guess in that neither account has them immediately hurrying off to Bethlehem, and in that they are likely to have married before Mary publicly appeared to be pregnant. The Church celebrates the espousal of Mary and Joseph on January 23rd, which would put a July or August wedding celebration nicely within the three to twelve months normally observed.
A Homily of St. Jerome, Priest