Terce, or Third Hour [Latin, from hora tertia, third hour after dawn], is a fixed hour for prayer dating back to Apostolic times. The third hour was also the traditional hour for Eucharist on feast days. This custom derived from the biblical reference to the third hour as the time that the holy spirit descended on the apostles in the form of a flame. The third hour is also called hora aurea or hora sacra ("golden hour" or "holy hour").
Beginning with the earliest mythologies, fire has represented spirit, soul, transformation and emerging consciousness. There are many ways that fire as a symbol manifests in world mythology, including fire-based creatures, gods, myths and rituals.
Whe the day of Pentecost came, they were all together in one place. Suddenly a sound like the blowing of a violent wind came from heaven and filled the whole house where they were sitting. They saw what seemed to be tongues of fire that separated and came to rest on each of them. All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Acts 2:1-4 [NIV]