Anytime to visit Manali is a good time as long as the roads are not blocked. I have personally spent the time around New Year in Manali twice and it's an absolute delight. The top can range anywhere between -15° to 3° or 4° Celsius so one should carry proper clothing. The Winter Carnival is also going on during this time and that makes the area around the market very lively. There is a good chance that you experience snow fall too! Adventure Sports and other activities can be enjoyed at Solang Valley at this time as the roads to Gulaba are closed.