Weather and when to go...hmm....that depends a lot on your personality.  If you hate crowds, you'd probably be best to stay away in the peak season which is May to September.  Niagara Falls is very crowded within those months, because it is absolutely beautiful.  The flowers are in bloom.  The parks are beautifully kept, and you can walk for hours without freezing your buns off.  Nothing feels better than the cool mist of the Falls falling on your sweaty body on a hot July afternoon.  Many of the tourist attractions are closed during the off season  as well (eg. Maid of the Mist and Whirlpool Boat Tours).  Also, many of the hiking trails are just too wet and muddy to enjoy in the winter and very early spring. 

 However, winter has its benefits too.  The Winter Festival of Lights is breathtaking, especially after a fresh snowfall.  If you can make it to Niagara Falls around Christmas or New Years, it's a very worthwhile visit.  There is usually a free, awesome New Years Eve FREE concert in Victoria Park, right across the road from the Falls.  Last year, Great Big Sea was the main event, and the year before that it was Smashmouth.  Imagine seeing those bands for FREE.   It's quite busy here on New Years Eve because the event is televised all across Canada.  

 So...you can see, both summer and winter have a lot to offer.  February is great, because it has Valentine's Day, and what can be more romantic than snuggling up with your significant other on a horse and buggy ride along Niagara Parkway in front of Niagara Falls? 

October and March are probably the slowest months, without much activity going  on.  If you prefer less crowds, those are definitely the months to visit.  If you enjoy the hustle and bustle of activity the summer months are the best. Although many people have not seen Niagara Falls in the winter that is also a good time to go, the falls freeze as the ry to hit the bottom of the falls. The best place in ontario is Niagara Falls, Ontario, Canada.