The best time for Chadar as per weather and other conditions is the 2nd week of January. There are many factors on which a smooth and risk free trek depends. How has the water frozen through out the river till Nerak Water fall, the no of people in your group, cold climate (temperature), etc etc. A lot of these factors favour January (exception being weather) 2nd week being the best. Ofcouse there is exception to the above too. Regards