These two saints were part of the group of the 12 apostles of Jesus. Simon received the nickname "Zealot" because he had belonged to a group of anti-Roman agitators before becoming an apostle. Judas, brother of Santiago and also called Thaddeus, asked Jesus after the Last Supper how Jesus himself could show himself as Messiah without using force.
Jude is called Thaddeus to differentiate him from Judas Iscariot who was the one who betrayed Jesus. In recent years, devotion to Saint Jude has become more popular than to Saint Simon, although their feast is celebrated on the same day because, according to an ancient tradition, the two always went together to preach the Word of God. Both were called by Jesus to be part of his group of 12 apostles. Both received the Holy Spirit in the form of tongues of fire on the day of Pentecost and witnessed the miracles of Jesus in Galilee and Judea and heard his teachings; they saw him already risen and spoke with him after his death on the Cross, they saw him risen and witnessed his ascension to heaven.