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.

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.

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.

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