June - August if you would like to ensure a spot on one of the boat tours, winter will bring its own stunning views if you don't mind bundling up. Snowfall in the area can come as early as mid-October. January is a good time to see the falls frozen.
Niagara Falls is the collective name for three waterfalls that straddle the international border between the province of Ontario (Canada) and the state of New York (United States), that form the southern end of the Niagara Gorge. From largest to smallest, the three waterfalls are Horseshoe Falls, the American Falls and Bridal Veil Falls. Horseshoe Falls lie mostly on the Canadian side and the American Falls entirely on the American side, separated by Goat Island.
Still wildly popular today, the falls first became a tourist destination during the 18th Century, by Mid-Century, it was the area's main industry. Napoleon Bonaparte's brother, Jérôme, visited with his bride in the early 19th Century. There are countless interesting and detailed stories of unusual occurrences, daredevils wild adventures plunging into the falls, heart-stopping tales of near-death experiences, and heroic rescues of people being pulled right from the water's edge moments before tragedy. While the 20th Century story of the falls was largely about efforts to harness the water's potential source of power, today, roughly eight million visitors a year (those numbers are just the US side) create revenue from the spectacle of the falls that warrant a commitment by the Canadian and American governments to maintain ongoing preservation efforts.
The most popular thing to do there is undoubtedly taking one of the boat rides on Maid of the Mist. Generations of visitors have been donning blue ponchos and getting drenched as you are taken into the heart of the falls. If you don't believe you could get any more wet, visit Cave of the Winds for an up-close view of the falls rushing waters - so close it is actually hard to keep your eyes open for more than a moment. There you will truly experience what being soaked to the bone feels like. If you are visiting when the boats are running, expect a long wait in line, but you can plan on feeling that you chose wisely after your falls experience.
If you are planning a trip to the falls, also make sure to look up on When to be Where's site the entry for the Niagara Whirlpool. it's not far from the falls and will give you a more intimate look at the powerful water and with virtually no tourists, incorporating a hearty hike down to the river.
Keep in mind you can visit the falls from either the American or the Canadian side. The link below is for American side tourist information.