Autumnal means relating to autumn, or typical of that season — the light, the weather, the colours. Autumnal leaves are red, gold, and brown. An autumnal evening is cool and a bit early-dark. It is a little more formal and writerly than just putting autumn in front of a noun. In everyday talk, people often say autumn colours or, in American English, fall weather, and save autumnal for writing, weather reports, and shops selling candles. Though actually, it can also name a feeling, not only a date on the calendar: an autumnal mood is quiet, a bit sad, a bit golden. If you only need “of that season,” the plain word autumn is always safe and never sounds stiff.