From the Treasury of Daily Prayer:

In the lists of the twelve apostles (Matthew 10:2-4; Mark 3:16-19; Luke 6:14-16; Acts 1:13), the tenth and eleventh places are occupied by Simon the Zealot (or “Cananaean”) and by Jude (or “Judas,” not Iscariot but “of James”), who was apparently also known as Thaddaeus. According to early Christian tradition, Simon and Jude journeyed together as missionaries to Persia, where they were martyred. It is likely for this reason, at least in part, that these two apostles are commemorated on the same day. Simon is not mentioned in the New Testament apart from  the lists of the twelve apostles. Thus he is remembered and honored for the sake of his office, and thereby stands before us–in eternity, as in his life and ministry on earth–in the name and stead of Christ Jesus, our Lord. We give thanks to God for calling and sending Simon, along with Jude and all of the apostles, to preach and teach the Holy Gospel, to proclaim repentance and forgiveness, and to baptize in the name of the Father and of the Son and of the Holy Spirit (John 4:1-2; Matthew 10; 28:16-20; Luke 24:46-49).

Jude appears in John’s Gospel (14:22) on the night of our Lord’s betrayal and the beginning of His Passion, asking Jesus how it is that He will manifest Himself to the disciples but not the world. The answer that Jesus gives to this question is a pertinent emphasis for this festival day: “If anyone loves Me, he will keep My word, and My Father will love him, and We will come to him and make Our home with him” (John 14:23). Surely both Jude and Simon exemplified, in life and death, their love for Jesus and their faith in His Word. Not only are we thus strengthened in our Christian faith and life by their example, but, above all, we are encouraged by the faithfulness of the Lord in keeping His promise to them to bring them home to Himself in heaven. There they live with Him forever, where we shall someday join them.