The Left Hand Of Darkness by Ursula K LeGuin

They were androgonist , they could choose become a male or female only part of the month.

"The inhabitants of Gethen are ambisexual humans; for twenty-four days (somer) of each twenty-six-day lunar cycle, they are sexually latent androgynes. They only adopt sexual attributes once a month, during a period of sexual receptiveness and high fertility, called kemmer. During kemmer they become sexually male or female, with no predisposition towards either,[23] although which sex they adopt can depend on context and relationships.[6] Throughout the novel Gethenians are described as "he", whatever their role in kemmer. This absence of fixed gender characteristics led Le Guin to portray Gethen as a society without war, and also without sexuality as a continuous factor in social relationships.[23][22] On Gethen, every individual takes part in the "burden and privilege" of raising children, and rape and seduction are almost absent."