September is one of the best months for getting dressed in. December is number one, because we get to dress for parties. July and August are usually excellent because of kaftans and bikinis. November is deeply satisfying, because of coats and boots. But September has the best outfits. September is the moment in the fashion calendar when new trends happen. The preceding months have a lovely looseness to them wherever you are, which means things get a bit feral by the end of August, even if the wildest adventure of your season is a face-off over the last space in a National Trust car park. But September is order and calm and routine. It works as a kind of fashion new year, because all those years of Back To School hardwire in us the expectation that this month means stiff new shoes and a shiny, unfamiliar version of ourselves reflected in the mirror. This September doesn’t have a great big bonkers trend such as puffball skirts or panama hats or tie-dye. That would be too much of a stretch at this point. Instead, the trend of this season is the return of fashion itself. As in, dressing in outfits rather than just covering yourself up with clothes. A sweater vest is a good place to start. It is fashion – you can tell, because we’re calling it a sweater vest rather than a tank top – but it is functional as well. Think of it as an accessory that keeps you warm. Wear it with any outfit that has long sleeves and a light fabric on the top half: a skirt with a blouse, say, or a long-sleeved dress. Being solid and boxy in shape, the sweater vest defines the silhouette and smartens you up. Ways to wear it include, but are not limited to: first, making a two-decades-old leopard-print skirt (which the wearer is sentimentally fond of) look friendlier and a bit less alarmingly over the top as daywear. Second, sharpening up a floaty dress which isn’t quite warm enough for the time of year but would look droopy under a cardigan. Third, layering over jeans worn with a fine-knit polo neck, which is a combination I like but find often doesn’t quite work at the waist. There is only one hard and fast rule, which is that you can wear it only over long sleeves, because a sweater vest over short sleeves is Florida golf club, and not in a good way. It is September, which means that these things matter once again. Time to bring your A game. Jess wears crochet vest, £45, warehousefashion.com. Other clothes, her own. Styling: Melanie Wilkinson, assisted by Peter Bevan. Hair and makeup: Joe Pickering at Carol Hayes Management