Niagara Falls is a year-round destination, but the experience changes dramatically with the season. June through August is peak: warm weather, full Hornblower boat operations, nightly illuminations, and Friday and Sunday fireworks. Expect lines at the falls and book hotel pickups early.
Late September and October are arguably the sweetest spot — fall colours along the Niagara Parkway, fewer tour buses, and Hornblower still running until late October. November through March the boats stop, but the Journey Behind the Falls and Skylon Tower remain open, and ice formations on the American Falls can be spectacular.
April and May bring tulips, wine-country bud-break, and quieter mid-week tours. If you want photographs without crowds, target a Tuesday or Wednesday morning in late September. If you want the full sensory experience — mist on your face, the boat horn, fireworks over the gorge — come in July.