Anything autumnal has to do with autumn: the fall season. Pumpkins are a popular autumnal food.
This word has to do with anything related to or associated with autumn. Sweater weather is autumnal. Pumpkin spice tea is autumnal. Leaves changing colors are autumnal. Halloween is an autumnal holiday. If you feel a chill in the air in August, you could say that's a sign of autumnal weather coming. Because of the mild weather, many people enjoy autumnal weather more than any other season's weather.