Osaka is a food city in every month. The castle lawn is the seasonal argument.
Late March–May and October–November
Blossom at the castle, milder walking, the same Dotonbori. Book rooms if your Japan trip is already sakura-peaked in Tokyo and Kyoto.
June–August
Humid. Eat after dark. The metro is air-conditioned. This is still a good food stop on a summer Honshu circuit.
December–February
Cool, cheaper, illuminations. Kuromon still opens. A practical add-on after Kyoto temples.