This is the second most important miracle in all of human history, after Christ's birth. Here he is raised from the dead. We know that if we follow him and take up our cross, the resurrection awaits every one of us as well.
Jesus rose up to heaven in sight of the apostles. The church as we know it didn't exist yet and they were still afraid. They thought he had left them and they didn't understand his promise of an advocate which was the Holy Spirit.
This is the third most important miracle in all of human history. Pentecost was the day our church was founded. God sent the Holy Spirit and timid frightened Apostles became fearless advocates who were able to bring many followers to Christ.
Mary went up to heaven in a way very similar to Elijah. She was taken up bodily.
Mary is our oldest sister in Christ. She became the first Christian when she answered the angel Gabriel "Let it be done to me according to God's will" when he asked her to become the mother of Jesus despite the danger. She lived in a time when a pregnancy outside of marriage could lead to death by stoning. Mary is the first Christian to be crowned in heaven. She is Queen of heaven.