The Catholic Church granted Matthew sainthood because of his duties as a public official. He is the Church's patron saint of tax collectors, accountants, civil servants and anyone who serves the government in some way.
Matthew was born sometime in the first century in Palestine. He was originally referred to as Levi or the son of Alphaeus in the Bible and most likely was given the name Matthew when he became one of Jesus' followers. He was a tax collector for the Romans and was not popular with the general public or the Pharisees, who were a class of Jewish people. Because of his position, he was not permitted to worship in the synagogue or marry a Jewish woman and was generally looked down upon by the rest of society.
When Jesus came to visit Matthew as he was carrying out his tax collector duties, he simply said "Follow me," and Matthew did so without hesitation. He became one of Jesus' 12 apostles and wrote the first book of the New Testament.
The Catholic Church has a liturgical calendar that has specific days which are set aside to honor different saints. Saint Matthew's date on that calendar is September 21. The church celebrates the day by singing, music and scripture relating to him in the church service.