"Light is the left hand of darkness and darkness is the right hand of light. Two are one, life and death, lying together like lovers in kemmer, like hands joined together, like the end and the way."
A 1969 Science fiction novel from Ursula K. Le Guin telling the story of Genly Ai of Earth, the Envoy of the Ekumen, to the planet Gethen. Gethen is far away from the rest of the planets humans were settled on by the ancient Hain. Gethen, which has been nicknamed by Terrans Winter, is a cold, glacial planet, but its real peculiarity is in the people: Gethenians are Gender Benders, and it has a profound effect on their society.Genly Ai's task is to persuade Gethenians to join the Ekumen, but they're unreceptive, while tensions between the nations of Orgoreyn and Karhide grow and the first war ever on Gethen is looming on the horizon...
Alternative Calendar: Karhide uses a calendar in which the current year is always "Year One", with only years before and after being numerated. It is a bit problematic, thus they refer to well-known events (like kings' ascension to throne) to record history. The followers of Yomeshta religion, however, have a calendar of the usual type.
Asexuality: Gethenians when not in kemmer are not only sex/gender-neutral, but also effectively asexual - and only about 1/4 of (adult) life is spent in kemmer. Kemmer, on the other hand, is a rather more desperate state than humans normally experience, to the point where it automatically gets you off work.
Deliberate Values Dissonance: Gethenians see no problem with sibling-sibling incest - while in "kemmer" a Gethenian is in an almost-Mate or Die situation, so they're forgiving when you're a horny teenager and the only also-in-kemmer person around is your sibling (though monogamous incest is seen just as we see it, and the lovers are expected to part ways when the first kid appears). On the other hand, theft is a horrible crime, since on an Ice World you may just be sentencing someone to death by starvation, and a suicide is seen with distaste as a kind of cowardice, an act of refusal to face reality.
On a personal level, Genly considering some things feminine in a rather derogatory way (see for example Multitasked Conversation below). Le Guin is a well-known feminist, and the main character remains sympathetic rather than being a Straw Misogynist.
Evil Chancellor: Zigzagged. At first, the Chancellor is a decent guyperson serving the insane King, but is soon replaced by a properly evil one. Of course, both the insane king and Genly initially think Estraven is evil.
Enigmatic Minion: Genly's view of Estraven is pretty close to this for most of the book
Fantastic Sexism: The Gethenians' reaction to Genly's permanent maleness, and, conversely, his reaction to their gender bending. Some Gethenians are permanently male or female, but this is regarded much as intersex conditions typically are in our society - worse, in fact, since generally more obvious.
Such "perverts" do however play an essential role in the Handdara religion due to their role in foretelling, as do the mentally ill: a Handdaran foretelling is made by a group of nine which includes "perverts", celibates, "zanies", at least one person in kemmer, and the "weaver", who joins their psychic energy together.
First Contact: Genly is not the first alien on Gethen (the Ekumen always sends explorers first, where possible), but he is the first official one, and the first envoy, complete with, yes, I Come in Peace and Take Me to Your Leader, both of which get rather... complicated by the local reception.
Gender Bender: Gethenians have a 26-day long cycle of going into heat ("kemmer") and spending the rest as asexuals. Which sex they assume is beyond their control, though the use of artificial chemical means is implied to be on the rise.
Gender Neutral Writing: Much pain to anyone not a Gethenian. Sooner or later they all decide to use "he" or "she" for convenience, though it leads to traps - Genly's feminine "landlady", whom he couldn't resist to think of as a woman, was a father several times and a mother not once.
On the other hand, Gethenians have no proper word for man/woman, so outsiders attempting to make themselves understood must use the words that refer to the male and female forms of kemmer (the only alternative being words for male and female animals). In other words, they can only make the sex/gender distinctions humans routinely use by thinking of them as animals in permanent heat. This creates a few difficulties for Genly.
Ikea Erotica: Most of the discussion of Gethenian sexuality; justified in that these are generally anthropological in intent. Also Genly's speculation about the possibility of intercourse with a Gethenian.
Medieval Stasis: Gethenians have known radio and electric automobiles for two thousand years, and they still use them. It's discussed in-universe whether it's the need to survive in harsh environment that uses up all the innovativeness.
Perfect Pacifist People: Sort of. Gethenians do kill each other by murders or clan feuds, but have never had a war. Like in the Medieval Stasis example, it's discussed if it's the harsh weather, or the lack of male-female — the most primal kind of "us versus them" — distinction. Later short stories reveal that the war will happen.
Shown Their Work: le Guin's interest in Taoism fairly clearly colours the descriptions of Gethenian religion, at least as far as Handdara is concerned: it is less clear what the breakaway Yomeshta group (dominant in Orgoreyn, and increasingly in Karhide) corresponds to, but at least it's a founder religion like Islam or Buddhism.
Telepathy: The peoples of the Ekumen have learnt to use this (which makes them Telepathic Spacemen to the Gethenians). The Gethenians have not, though it's clear they are capable of learning (as are humans, such as Genly).