A preacher from the hardline Salafist movement that has achieved growing prominence in Tunisia since the 2011 Arab Spring uprisings told Tunisians on Monday that the exchange of New Year's greetings was un-Islamic.
"Sharing the feast days of the infidel or even sending them greetings to mark them is a big sin," Sheikh Beshir Ben Hassine said in sermon posted on Facebook.
"Wishing someone a Merry Christmas or a Happy New Year is forbidden by Islam," the preacher added.
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The regime of veteran strongman Zine El Abidine Ben Ali, overthrown in the first of the revolts that rocked the Arab world in 2011, was defiantly secular but, since its overthrow, Islamists have increasingly taken centre stage.
Islam has its own calendar that dates events from the 7th Century flight of the Prophet Mohammed's forces from Mecca to Medina before their eventual victorious re-entry into the holy city.
Members of Tunisia's militant Salafist movement, thought to number between 3,000 and 10,000, have been implicated in numerous acts of violence since Ben Ali's ouster.
On September 14, hundreds of radical Islamists angered by a US-made film mocking their religion attacked the US embassy in Tunis and a neighbouring American school in a day of violence that left four people dead and dozens wounded.
Two Islamists arrested in connection with the embassy attack and who were on hunger strike died last month.