Six people arrested in December protests will be put to death, Iranian authorities announced Monday, in what appeared to be strong warning to the opposition ahead of a traditional annual celebration.

The tradition, the Feast of Fire, goes back thousands of years to Zoroastrian times and has been banned in Iran in recent decades because of its non-Islamic roots. The opposition had called for its celebration this year as a sign of protest.

The supreme leader, Ayatollah Ali Khamenei, issued a decree saying the feast “has no religious basis and is harmful and must be avoided,” the government Web site reported.

The celebration includes jumping over fires in the evening, followed by the Iranian version of trick-or-treating, when young people cover themselves head to toe in chadors and bang spoons on pots as they knock on neighbors’ doors for candy. Two men who had also received death sentences for their part in the December protests were executed last month, just ahead of another holiday, the 31st anniversary of the Islamic revolution.