This year Jól starts today, December 21st 2017. Jól lasts for three nights* starting at the winter solstice. So, this time December 24 will be the first day of the new year - this date is not celebrated, and neither is the 25th, as these dates are just ordinary dates; not significant in any way.
The traditional greeting is simply a new year toast/greeting, meaning something akin to "live long and peacefully", literally "for (the) year and peace". In Iceland it is still a common new years greeting. Here it is:
Til árs ok friðar