夏至是二十四节气中最早被确定的一个节气。公元前七世纪，先人采用土圭测日影，确定了夏至。每年夏至从6月21日（或22日）开始，至7月7日（或8日）结束。《恪遵宪度抄本》记载：“日北至，日长之至，日影短至，故曰夏至。至者，极也。”
《礼记》曰：“夏至到，鹿角解，蝉始鸣，半夏生，木槿荣。”
夏至分为三候：“一候鹿角解；二候蝉始鸣；三候半夏生。”糜、鹿虽属同科，但古人认为二者一属阴一属阳。鹿的角朝前生，所以属阳。夏至日阴气生而阳气始衰，所以阳性的鹿角便开始脱落。而糜因属阴，所以在冬至日角才脱落；雄性的知了在夏至后因感阴气之生便鼓翼而鸣；半夏是一种喜阴的药草，因在仲夏的沼泽地或水田中出生所以得名。在炎热的仲夏，一些喜阴的生物开始出现，而阳性的生物却开始衰退了。
北魏崔灵恩《三礼义宗》解释为：“夏至为者，至有三义；一以阴阳气之至极，二以明阳气之始至，三以明日行之北至。故为至。”夏至日这天太阳到达了最北端，阳光几乎直射北回归线，北半球白天最长，黑夜最短；过了夏至日，阳光直射位置逐渐向南移动，白天开始一天比一天缩短。
夏至，古时称“夏节”、“夏至节”。古时夏至日，人们通过祭神祈求灾消年丰。《周礼·春官》记载：“以夏日至，致地方物魈。”周代夏至祭神，意为清除疫疠、荒年与饥荒死亡。《史记·封禅书》则云：“夏至日，祭地，皆用乐舞。”唐《酉阳杂俎·礼异》“夏至日，进扇及粉脂囊，皆有辞。”宋朝在夏至之日始，百官放假三天；清朝又是“夏至日为交时，日头时、二时、末时，谓之‘三时’，居人慎起居、禁诅咒、戒剃头，多所忌讳……”
在朝廷，“夏至”之后，皇家则拿出“冬藏夏用”的冰“消夏避伏”，从周代始，历朝沿用，竟而成为制度。
民间还流传着一些关于夏至的谚语：“吃过夏至面，一天短一线”、“夏至馄饨冬至团，四季安康人团圆”、“夏至杨梅满山红，小暑杨梅要出虫”、“芒种玉秧放庭前，夏至稻花如白练”……
我们小时曾念过“一九二九不出手”的冬至九九歌，其实，夏至也有九九歌，夏至后，第三个庚日至第四个庚日的十天为初伏，第四个庚日至立秋后初庚的十天为中伏，立秋后初庚起的十天为末伏，九九歌形象地描述了入伏后从炎炎酷暑到逐渐秋凉的天气变化：
《豹隐纪谈》--夏至九九歌
[宋]周遵道
一九至二九，扇子不离手；
三九二十七，吃茶如蜜汁；
四九三十六，争向街头宿；
五九四十五，树头秋叶舞；
六九五十四，乘凉不入寺；
七九六十三，入眠寻被单；
八九七十二，被单添夹被；
九九八十一，家家打炭壑。
《吴下田家志》--夏至九九歌
[南宋]陆泳
夏至入头九，羽扇握在手；
二九一十八，脱冠着罗纱；
三九二十七，出门汗欲滴；
四九三十六，卷席露天宿；
五九四十五，炎秋似老虎；
六九五十四，乘凉进庙祠；
七九六十三，床头摸被单；
八九七十二，子夜寻棉被；
九九八十一，开柜拿棉衣。”
说了跟夏至有关的许多话题，话说在汉朝的某次夏至日宴饮上还曾诞生过一个成语，这个成语家喻户晓，生活中也时常会用到，下面来一起阅读吧\(≥▽≤)/
[汉]应劭《风俗通义·怪神·世间多有见怪惊怖以自伤者》：
“予之祖父郴为汲令，以夏至日请见主簿杜宣，赐酒。时北壁上有悬赤弩，照於杯中，其形如蛇。宣畏恶之，然不敢不饮，其日便得腹腹痛切，妨损饮食，大用羸露，攻治万端，不为愈。後郴因事过至宣家闚视，问其变故，云畏此蛇，蛇入腹中。郴还听事，思惟良久，顾见悬弩，必是也。则使门下史将铃下侍徐扶辇载宣於故处设酒，杯中故复有蛇，因谓宣：‘此壁上弩影耳，非有他怪。’宣意遂解，甚夷怿，由是瘳平。”
这个故事大意是，客人见杯中弓影，以为蛇在酒中，勉强喝下。后疑虑生病，明白真相后，疑虑消失不治而愈。后用杯弓蛇影或弓影杯蛇，指因错觉而产生疑惧，比喻疑神疑鬼，妄自惊忧。
清人黄遵宪《感事》诗：“金玦庞凉含隐痛，杯弓蛇影负奇冤。”赵翼《七十自述》诗：“水中见蟹犹生怒，杯底适蛇得不惊。”
Nearly 2,000 years ago, during the Han Dynasty, there was a county magistrate called Ying Chen. One summer day, he invited his secretary Du Xuan to his house and treated him with wine. On the north wall of the room hung a red bow. It was reflected in Du Xuan's cup. Du Xuan took the reflection for a squirming snake. He was very frightened but he dared not turn down Ying Chen's offer because Ying was his superior. He had to swallow the wine with his eyes closed.
When he was back at home he felt so painful in his chest and stomach that he could hardly eat and drink any more. He sent for the doctor and took some medicine, but nothing could cure him. When Ying Chen asked Du Xuan how he got so seriously ill, Du told him he drank the wine with a snake in his cup the other day. Ying Chen found something strange about that. He returned home , thought hard, but he could not find an answer. Suddenly the shadow thrown by the bow on the wall caught his eye. "That's it!" he shouted. He immediately sent his man to fetch Du Xuan. He seated him where he sat before and offered him a cup of wine. Du Xuan saw the snake-like shadow again. Before Du was scared out of his wits again, Ying Chen said , pointing at the shadow, "The 'snake' in the cup is nothing but a reflection of the bow on the wall!" Now that Do Xuan knew what it was, he felt much easier. And strange enough, his illness disappeared the next moment.This story was later contracted into the idiom "Bei Gong She Ying'. It is used to describe someone who is very suspicious. It is like saying someone is afraid of his own shadow.
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