For many the summer might not be the knitting high season but there is no reason for that. There are lots of yarns which are really nice to work with on a hot summer day. Here is a list of the best ones.
Cotton is lovely to wear on a hot summer day. It keeps you cool because it transports heat away from the body, a characteristic repeats itself in organic yarns, but which stands in contrast to synthetic yarns, such as acrylic. In addition, cotton is easy to wash. This is important, as you typically need to wash summer knitwear somewhat more often than winter knitwear. When knitting in cotton, be aware that it can tend to get wrinkled and in some cases become a little stiff after several washes. It can also be a bit hard on the hands to knit in cotton compared to wool yarns.
A good alternative to the pure cotton is a mixture of wool and cotton. Here you get the best of both worlds. You get many of the wool's good temperature-regulating properties (see, for example, Merci from Filcolana (affiliate link)). This mixture is a summer yarn which is easy to knit in, not too hard on the hands and gives a nice and regular pattern. The cotton ensures you don't get too hot. If you choose a yarn where the wool is superwash, it is also very easy to wash (see, for example, Cotton Merino from Drops (affiliate link), in which the Summer Snow Top is knitted)
If you're looking for a transitional sweater or cardigan, wool/cotton blends go really well with a thin fluffy silk-mohair or silk-alpaca. Here you get a less warm sweater that is still super soft and has a nice furry feel. Here we have really good experiences with Esther from Permin, which is not so soft in itself, but really delicious in the combo (affiliate link).
Silk is an incredibly durable natural material, which also has good temperature-regulating properties. There are different silk yarns. Here is a small selection.
The smooth mulberry silk is typically slightly shiny and soft against the skin. It has many of the same good properties as cotton, and - unlike cotton - settles into an even stitch pattern after washing. However, be aware that the silk tends to stretch (sometimes a lot) over time. It can be hard on the hands to knit in and requires a little extra attention to catch all the threads while you knit, as they often consist of many thin threads. The silk yarns are super exclusive and therefore relatively expensive.
Bourette silk has a coarser texture and often a characteristic scent. The properties of raw silk are reminiscent of wool in many ways and give a slightly rustic look. See for example, Soft silk from BC Garn (affiliate link) or Pure Silk from Knitting For Olive.
Yarns that mix silk and other fibres are often less hard on the hands and a little cheaper. However, perhaps also a little less summer-friendly - depending on what it is mixed with.
A cheap and soft alternative that also lays down nicely when it is washed. It feels very smooth and soft against the skin, but somewhat harder for the hands to knit in than wool yarns - in the same way as silk. It is a natural yarn that feels cool against the skin on a hot summer day, as it carries the heat away from the body. The yarns are often available in very thin qualities, so you can make your summer top super thin if you wish (see, for example, Bambino from Viking Garn (affiliate link), which the Summer Sun Top is knitted in). As with the fine mulberry silk, the yarn often consists of many thin threads, which can make it difficult to get all the threads together every time - it therefore requires increased attention or a certain acceptance of imperfections when knitting in bamboo-cotton.
Linen is an organic material, it has a very rustic look and is super durable. On the other hand, it can also become stiff after many washes. Linen is also found in many exciting mixed yarns, which get the good properties of linen.
Suggestions of pure linen yarns are Hør Organic from Isager or the cheaper Lino from BC Garn - alternatively Line from Sandnes or Belle from Drops if you are looking for linen blends (affiliate links).