An anniversary (from the Latin anniversarius, from the words for year and to turn, meaning (re)turning yearly; known in English since c. 1230) is a day that commemorates and/or celebrates a past event that occurred on the same day of the year as the initial event. For example, the first event is the initial occurrence or, if planned, the inaugural of the event. One year later would be the first anniversary of that event. The word was first used for Catholic feasts to commemorate saints.

For monthly recurrences, one might say mensiversary, from the Latin word mensis, for month, and versus, meaning to (re)turn. Mensiversary was used in a 1925 Time magazine article, titled 18th Mensiversary.

Anniversary names

• Birthdays (v.) are the most common type of anniversary, where the birth date of a person is commemorated annually. The actual celebration is sometimes moved for practical reasons, as in the case of an official birthday.
• Wedding anniversaries are also often celebrated on the same day of the year as the wedding occurred.
• Death anniversary.

The Latin phrase dies natalis (literally birth day) has become a common term, adopted in many languages, especially in intellectual and institutional circles, for the anniversary of the founding ("legal or statutory birth") of an institution, such as an alma mater (college or other school). Even in ancient Rome, we know of the 1 Aquilae natalis ("birthday of the eagle", anniversary of the official founding of a legion).

Most countries around the world celebrate national anniversaries, for example the United States Bicentennial. These could be the date of independence of the nation or the adoption of a new constitution or form of government. The important dates in a sitting monarch's reign may also be commemorated, an event often referred to as a 'Jubilee'.

Anniversaries of nations are usually marked by the number of years elapsed described with Latin words or Roman numerals.

Latin-derived numerical names

The root elements of each word are literally multiplied together to create the anniversary name. For example, the word sesquicentennial (an anniversary of 150 years) is broken down as sesqui- (1½) x centennial (100 years). Sometimes new anniversary names are coined incorrectly by adding the root elements rather than multiplying them, with unfortunate results.

• Annual - 1 year
• Biennial - 2 years
• Triennial - 3 years
• Quadrennial - 4 years
• Quinquennial - 5 years
• Sexennial - 6 years
• Septennial - 7 years
• Octennial - 8 years
• Novennial - 9 years
• Decennial - 10 years
• Undecennial - 11 years
• Duodecennial - 12 years
• Tredecennial - 13 years
• Quattuordecennial - 14 years
• Quindecennial - 15 years
• Vigintennial or vicennial - 20 years
• Semicentennial or quinquagenary - 50 years
• Semisesquicentennial - 75 years
• Variations: Demisesquicentennial or hemisesquicentennial
• Centennial - 100 years
• Quasquicentennial - 125 years
• Sesquicentennial - 150 years
• Demisemiseptcentennial or quartoseptcentennial - 175 years
• Note: Terquasquicentennial is a coined word for an anniversary of 175 years, but the elements of the word literally refer to an anniversary of 375 years, as follows: ter- (3) x quasqui- (1¼) x centennial (100 years)
• Note: Septaquintaquinquecentennial is a coined word for an anniversary of 175 years, but the elements of the word literally refer to an anniversary of 35,000 years, as follows: septaquinta- (70) x quinque- (5) x centennial (100 years)
• Bicentennial - 200 years
• Semiquincentennial - 250 years
• Variation: Bicenquinquagenary was used by Princeton University in 1996, and by Washington and Lee University in 1999.
• Tercentennial or tricentennial - 300 years
• Semiseptcentennial - 350 years
• Quadricentennial or quatercentenary- 400 years
• Quincentennial - 500 years
• Sexcentennial - 600 years
• Septcentennial or septuacentennial - 700 years
• Octocentennial - 800 years
• Nonacentennial - 900 years
• Millennial - 1000 years
• Bimillennial - 2000 years