The short answer
April for cherry blossoms at Osaka Castle · October for food festivals.
Why Osaka over Tokyo or Kyoto?
Osaka is Japan's culinary capital — takoyaki, okonomiyaki, kushikatsu were all invented or perfected here. Cost-of-living is meaningfully lower than Tokyo. The city has a more casual, energetic feel than Kyoto. Most travelers pair Osaka with Kyoto (30 min by train) and Nara (45 min) on a Kansai loop.
What to time around
- Tenjin Matsuri (July 24-25) is one of Japan's three biggest festivals — incredible if you're already there for summer, but otherwise summer heat is brutal.
- Universal Studios Japan Halloween (Sept-Nov) draws huge crowds — book accommodation outside Universal City for sanity.
- Cherry blossoms at Osaka Castle (April 1-8 typical peak) — one of the most photographed views in Japan.