The short written form of Sunday. Three letters and a dot — the dot is there to tell you letters have been cut off the end. You write it, you do not say it. When your eye meets 'Sun.' on a page, your mouth says 'Sunday'. This is true of all the day abbreviations: Mon., Tue., Wed., Thu., Fri., Sat., Sun.

of Sunday就是 Sunday 的 The short written form書面縮寫。Three letters三個字母 and a dot加一個點——the dot那個點 is there to tell you在那裡是要告訴你 letters字母 the end從字尾 have been cut off被剪掉了。You你 write it用寫的，you do not say it不用說的。When your eye當你的眼睛 on a page在頁面上 meets遇到 'Sun.'，your mouth你的嘴 says唸的是 'Sunday'。all the day abbreviations所有星期縮寫 This is true of都是如此：Mon., Tue., Wed., Thu., Fri., Sat., Sun.。

Where does it appear? Anywhere space is tight and the reader already knows what to expect: shop opening hours on a door, a calendar square, a train or bus timetable, a class schedule, a doctor's appointment card, the small print of a form. 'Open Mon.–Sat., closed Sun.' In writing where space is not tight — a letter, an essay, a message to a friend — write the whole word. An abbreviation in a normal sentence looks careless, like you could not be bothered.

Where does it appear它會出現在哪裡？space is tight空間很擠、and the reader already knows what to expect而讀者已經知道會看到什麼的 Anywhere任何地方：on a door門上的 shop opening hours商店營業時間、a calendar square日曆的格子、a train or bus timetable火車或公車時刻表、a class schedule課表、a doctor's appointment card醫生的預約卡、a form表單 the small print of的小字。'Open Mon.–Sat., closed Sun.'週一到週六營業，週日公休。space is not tight空間不擠的 In writing書寫裡——a letter信件、an essay文章、a message to a friend給朋友的訊息——write the whole word就把整個字寫出來。in a normal sentence在正常句子裡 An abbreviation用縮寫 looks careless看起來隨便，like就像 you could not be bothered你懶得好好寫。

One thing worth knowing: the dot is a British habit more than an American one, and even in Britain many people now leave it off. Calendars and phones almost always print 'Sun' with no dot, and nobody thinks it wrong. So do not worry that a missing dot is a mistake — both are fine, and the dot is slowly disappearing.

One thing worth knowing有件事值得知道：the dot這個點 more than an American one與其說是美式、is a British habit不如說是英式習慣，and even in Britain而且連在英國 many people很多人 now現在 leave it off也不加了。Calendars and phones日曆和手機 almost always幾乎都 with no dot不帶點地 print印出 'Sun'，and nobody thinks it wrong也沒有人覺得錯。So所以 do not worry不用擔心 a missing dot少了點 is a mistake是個錯——both兩種 are fine都可以，and the dot而這個點 is slowly disappearing正在慢慢消失。

The capital S is not optional, though. Days of the week are always capitalised in English, whether written short or long.

though不過，The capital S大寫的 S is not optional可不是隨你便的。in English英文裡 Days of the week星期名稱 whether written short or long不管寫短寫長 are always capitalised都一定要大寫。