Chiffon is the fabric that makes a dress feel like a dress, light and a bit alive when you move, and that's exactly why I think it earns a place over the heavier stuff once the weather turns. The whole point of it is the drape and the slight see through quality, so you need to buy into both of those things rather than fight them, because a chiffon dress that's been made to behave like cotton has lost the reason you wanted it.
It is at its best for anything with a bit of occasion to it, a wedding where you'll be standing around for hours in the heat, a garden do, a long lunch that runs into evening, any date where you want movement when you walk rather than a dress that just hangs there. It also does well on holiday over a swimsuit once the sun's gone down and you want something that doesn't cling after a day on a sunbed. Where it struggles is anywhere practical. Wind turns a fuller chiffon skirt into a liability, cycling or running for a bus is a bad idea, and anything with small children or a lot of standing near a buffet table is asking for trouble because the fabric snags and marks more easily than it looks like it should.
Be careful what you put it with. Anything structured and stiff, a sharp blazer, a hard leather jacket, tends to fight the softness instead of complementing it, so you end up looking like two outfits stapled together. Heavy boots can do the same, dragging the whole thing down at the hem. And if the dress already has a slip built in, don't layer another slip underneath in a different shade, you'll see both of them through the skirt in certain light.
For fit, go up rather than down if you're between sizes, since chiffon doesn't stretch and a tight bodice will pull at the seams in a way that shows. Check the lining separately from the outer layer, they should move slightly independently rather than feel stuck together, and check the hem falls the same length all the way round, uneven hems are common on this fabric because it's slippery to cut. Raise your arms in the changing room before you decide, a lot of these dresses ride up at the waist or gape at the underarm once you move rather than when you're standing still.
Tomorrow morning I'd put mine with flat sandals, a denim jacket thrown over the top, and nothing else trying to compete.