New York City is wonderful to see year-round because you can experience all 4 seasons and there are always things going on. There really is no "best" or "worst" time to visit.
April-June bring a beautiful Spring season where the trees are in bloom and the city is starting to shine again. Temperatures are pretty mild during this time of year.
July-August can be extremely hot (temperatures down in the subway can be over 100 degrees!) in the city and bring a lot of tourists from all over. Locals usually escape to the suburbs or surrounding areas during this time of year to beat the heat.
September is usually either the extension of Summer or the beginning of Fall--you never really know what you will get. Indian Summers are pretty common in this city though.
October-November is the start of the Fall and theatre/ballet/concert season. It is a beautiful time in the city where the trees start to change foliage and there is a fresh, crisp taste in the air.
The city is in full holiday swing in December and there are so many things to do and see during the month (it is also not as cold yet). If you're into holiday festivities, definitely come during this time of year.
January-February are generally the slower times of the year because it traditionally brings extremely cold weather and the occasional blizzard. Tourist accommodations/prices are generally slightly lower at this time of year because of the low season traffic.
March is usually an extension of Winter/early start to Spring. If you want to beat the crowds, this is another good time of year to come through.
