Kayitz is the Hebrew word for summer. The word makes its appearance very early in  the Bible - in Genesis, right at the end of the account of the Flood. Noah and his family have just emerged from the ark and made an offering to God. Smelling the pleasing scent of the sacrifice, God resolves, "Never again will I doom the earth because of man... So long as the earth endures, seedtime and harvest, cold and heat, summer and winter, day and night, shall not cease" (Gen, 8:21).
The verse conveys the idea that kayitz is the opposite of winter, just as cold is the opposite of heat, harvest the opposite of seedtime, and the day the opposite of night. For, as seasons go, in this part of the world, here are basically only two - winter (horef), when it rains, and summer (kayitz), when it does not. Lengthy springs, autumns, and Indian summers have to be relegated to travel plans to other parts of the planet.
For the ancient Hebrews, kayitz was the time when the fruit ripened. Apples, pears, and oranges had not been introduced to the biblical world, and the main - and nearly only - fruits around were figs and grapes, which indeed ripen in summer, and accordingly were also called kayitz.A word that sounds similar to kayitz - katzeh (edge) - is unrelated linguistically, but you do get a sense of the shared implication of contrasts. "Make one cherub at end (katzeh) and [one cherub] on the other end," God instructs Moses with regard to the construction of the Ark of the Covenant (Exod. 25:19). If you are uncompromising in your attitudes, you are afflicted with kitzoniyut, and if you take these ideas to political extremes, you have become a kitzoni - an extremist. A group of extremists, left or right, is referred to as kitzonim.
Not far from katzeh is ketz - end, in the sense of cessation, as in ketz hayamim, the End of Days. If you are one of those who have tried to figure out when that time will come around, you are one of the mechashvei kitzin - end calculators. If there is no hope left, for the end of days or for any other matter, then kalu kol haketzim - all the ends have finished.
Ketz is not linguistically related to kayitz either. But it still seems as if there is a connection. As the summer heat sets in, it seems as if torrid days with scorching sunshine are all we can expect until the end of time. This hopeless feeling makes people less patient and much more extreme.
Whenever kayitz and horef appear together in the Bible, kayitz is always mentioned before horef. Winter always follows summer - the heat of kayitz will eventually cool off with the rain of horef. On that day thousands of years ago at the end of the Flood,  though God stated that "the devisings of man's mind are evil from his youth," He blessed Noah and his sons and told them (Gen.9:11), "Be fertile and increase, and fill the earth" - perhaps because the seasons change and reason then prevails.