Meskel, in the Ethiopian Orthodox Church, is an annual religious holiday commemorating the discovery of the True Cross by Queen Helena (Saint Helena) in the fourth century.
The Meskel celebration includes the burning of a large bonfire, or Demera, based on the belief that Queen Eleni had a revelation in a dream. She was told that she shall make a bonfire and that the smoke would show her where the true cross was buried. So she ordered the people of Jerusalem to bring wood and make a huge pile. After adding frankincense to it the bonfire was lit and the smoke rose high up to the sky and returned to the ground, exactly to the spot where the Cross had been buried.
According to local traditions, this Demera-procession takes place in the early evening the day before Meskel or on the day itself. The firewood is decorated with daisies prior to the celebration. Charcoal from the remains of the fire is afterwards collected and used by the faithful to mark their foreheads with the shape of a cross (compare Ash Wednesday). Edward Ullendorff records a number of beliefs of the meaning of Demera, with some believing that it "marks the ultimate act in the cancellation of sins, while others hold that the direction of the smoke and the final collapse of the heap indicate the course of future events -- just as the cloud of smoke the Lord over the Tabernacle offered guidance to the children of Israel (Exod. 40:34-38)."
One explanation for the high rank this festival has in the church calendar is that it's believed that a part of the true Cross has been brought to Ethiopia from Egypt. It is said to be kept at Amba Geshen, which itself has a cross shape.
Meskel occurs on 17 Meskerem in the Ethiopian calendar (September 27, Georgian calendar, or September 28 in leap years). "Meskel" (or "Meskal" or "Mesqel", there are various ways to transliterate from Ge'ez to Latin script) is Ge'ez for "cross".