That stretch between seasons, when it is not quite warm enough for nothing and not cold enough for a coat, is when a denim shirt earns its spot. I reach for mine on the days I cannot be bothered to think, over a dress that is slightly too summery for the weather, under a jacket when the wind picks up on the walk to the station. It is the one thing in my wardrobe that does not care what month it is.
If you own none, start with a mid blue, slightly worn in rather than stiff off the peg, in a cotton that has a bit of give to it. Avoid anything that feels like cardboard on the first try, because it stays that way, it does not soften with washing the way people promise it will. The difference between one that lasts and one that is in the donation bag by spring usually comes down to the weight of the fabric and how it is finished at the seams. Cheap ones fray at the collar points after a few washes and the buttons start to look like they came from a different shirt entirely, slightly too shiny, slightly too light. A well made one has proper flat felled seams you can feel under your finger, a collar that keeps its shape without interfacing that cracks, and buttons that sit flush rather than proud. Mine has been through more hot washes than I would admit to and the hem has not gone stringy once.
It sits well with almost everything that is not also denim, which sounds obvious until you have made that mistake standing in a lift wondering why everyone is looking at your legs. Tuck it into trousers with a blazer over the top and it reads as a proper outfit rather than an afterthought. Knot it over a plain top with jeans and it does something a cardigan never quite manages, it has a bit of structure to it. Worn open as a layer over a dress, it takes the edge off anything too fussy underneath.
On colour, true mid blue is the easiest one to live with, it does not date and it goes with the rest of your clothes without a fight. A black denim shirt looks sharp in the shop and then fades patchily at the elbows within a season, so go in expecting that if you choose it. Pale, almost bleached denim is lovely but shows marks the moment you sit near anything with a pen in it.
Tomorrow I am wearing mine knotted over black trousers, with trainers, because the meeting does not deserve more effort than that and the shirt will look fine regardless.