Anniversary
An anniversary is the date on which an event took place or an institution was founded in a previous year, and may also refer to the commemoration or celebration of that event. The word was first used for Catholic feasts to commemorate saints.
Most countries celebrate national anniversaries, typically called national days. These could be the date of independence of the nation or the adoption of a new constitution or form of government. There is no definite method for determining the date of establishment of an institution, and it is generally decided within the institution by convention. The important dates in a sitting monarch's reign may also be commemorated, an event often referred to as a "jubilee".
Names
 Birthdays are the most common type of anniversary, on which someone's birthdate is commemorated each year. The actual celebration is sometimes moved for practical reasons, as in the case of an official birthday or one falling on February 29.
 Wedding anniversaries are also often celebrated, on the same day of the year as the wedding occurred.
 Death anniversaries.
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). In ancient Rome, the [dies] Aquilae natalis was the "birthday of the eagle", the anniversary of the official founding of a legion.
Anniversaries of nations are usually marked by the number of years elapsed, expressed with Latin words or Roman numerals.
Numerical
This section possibly contains original research. (February 2018) 
Latin terms for anniversaries are mostly straightforward, particularly those relating to the first thirty years (1–30), or multiples of ten years (30, 40, 50, 60, 70 etc.), or multiples of centuries or millenniums (100, 200, 300, 1000, 2000, 3000, etc.). In these instances, the name of the anniversary is generally derived from the Latin word(s) for the respective number of years. When anniversaries relate to fractions of centuries (125, 150, 175, 225, 250, 275 years—i.e. 1.25, 1.5, 1.75, 2.25, 2.5, and 2.75 centuries), the situation is not as simple.
Roman fractions were based on a duodecimal system. From 1⁄12 to 8⁄12 they were expressed as multiples of twelfths (uncia "twelfth"; the source of the English words inch and ounce) and from 9⁄12 to 11⁄12 they were expressed as multiple twelfths less than the next whole unit—i.e. a whole unit less 3⁄12, 2⁄12 or 1⁄12 respectively. There were also special terms for quarter (quadrans), half (semis), and threequarters (dodrans). Dodrans is a Latin contraction of dequadrans which means "a whole unit less a quarter" (de means "from"; quadrans means "quarter"). Thus for the example of 175 years, the term is a quarter century less than the next whole (bi)century or 175 = (−25 + 200).
In Latin, it seems that this rule did not apply precisely for 1+1⁄2. While secundus is Latin for "second", and bis for "twice", these terms are not used such as in sesquisecundus. Instead sesqui (or ses) is used by itself.
Anniversary  Latinderived term  Other terms  Comments 

6 months  Semiannual  'Biannual' means twice in a year, but is also commonly used incorrectly to mean once every two years ('biennial').  
Biannual  
1 year  Annual  Paper  
2 years  Biennial  Cotton  'Biennial' means once every two years, but is also commonly used incorrectly to mean twice in a year ('biannual'). 
3 years  Triennial  Leather  
4 years  Quadrennial  Linen  
5 years  Quinquennial  Wood  
6 years  Sexennial  Iron  
7 years  Septennial  Wool  
8 years  Octennial  Bronze  
9 years  Novennial  Copper  
10 years  Decennial  Tin Aluminum 

Denary  
11 years  Undecennial  Steel  
12 years  Duodecennial  Silk  
121⁄2 years  Parsley  A humorous or mock wedding anniversary celebrated in Germany and the Netherlands where everyone needs to wear something green  
13 years  Tredecennial  Lace  
14 years  Quattuordecennial  Ivory  
15 years  Quindecennial  Crystal  
16 years  Sexdecennial  Sapphire  Sapphire is separately used for other anniversaries 
17 years  Septdecennial  Orchid  
18 years  Octdecennial  Quartz  
19 years  Novdecennial  Jade  
20 years  Vigintennial  China/Porcelain  
Vicennial  
Vicenary  
Bidecennial  
25 years  Quadranscentennial  Silver  
30 years  Tricennial  Pearl  
Tricenary  
35 years  Quintricennial  Coral  
40 years  Quadragennial  Ruby  
Quadragenary  
45 years  Quinquadragennial  Sapphire  
50 years  Semicentennial  Golden  Previously, "jubilee" by itself was used to indicate celebrations at 50 year intervals 
Quinquagenary  
55 years  Quinquinquagennial  Emerald  
Quinquinquagenary  
60 years  Sexagennial  Diamond  Diamond is separately used for the 75th anniversary, its use for 60th years being popularized by Diamond Jubilee of Queen Victoria 
Sexagenary  
65 years  Quinsexagennial  Sapphire  Sapphire is separately used for other anniversaries 
70 years  Septuagennial  Platinum  
Septuagenary  
75 years  Semisesquicentennial  Diamond  Diamond is separately used for the 60th anniversary. Semisesquicentennial can be broken down to understand its meaning: "semi"  half of + "sesqui"  in the ratio of 3:2 + "centennial"  100 years. Broken out mathematically, 1/2 * 3/2 * 100 = 75. 
Demisesquicentennial  
80 years  Octogintennial  Oak  
Octogenary  
90 years  Nonagintennial  Granite  
Nonagenary  
100 years  Centennial  Obsidian  
Centenary  
125 years  Quasquicentennial  Term is broken down as quasqui (and a quarter) centennial (100 years). Quasqui is a contraction from quadrans "a quarter" plus the clitic conjunction que "and". The term was coined by Funk and Wagnalls editor Robert L. Chapman in 1961.  
150 years  Sesquicentennial  Term broken down as sesqui (one and a half) centennial (100 years)  
175 years  Dodransbicentennial  Dodrans is a Latin contraction of dequadrans which means "a whole unit less a quarter" (de means "from"; quadrans means "quarter"). 175 years is a quarter century less than the next whole (bi)century (175 = 200 − 25).  
Dodrabicentennial  Alternative Latin form of Dodransbicentennial  
Dequasbicentennial  Alternative Latin form of Dodransbicentennial  
Dosquicentennial  Dosquicentennial has been used in modern times and this is perhaps a modern contraction of "dequadrans".  
Demisemiseptcentennial  Probably^{[attribution needed]} a modern coined term: demisemiseptcentennial; literally onehalf (demi) × onehalf (semi) × seven (sept) × 100 years (centennial)—also demisemiseptcentenary.  
Quartoseptcentennial  Probably^{[attribution needed]} a modern coined term: quartoseptcentennial; literally onequarter (quarto) × seven (sept) × 100 years (centennial)—also quartoseptcentenary.  
Terquasquicentennial  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) × quasqui (11⁄4) × centennial (100 years)  
Septaquintaquinquecentennial  Suggested by lexicographer Robert L. Chapman to William Safire; first appeared in Safire's column, "On Language" (The New York Times Magazine, February 12, 1995). It 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) × quinque (5) × centennial (100 years)  
200 years  Bicentennial  
Bicentenary  
225 years  Quasquibicentennial  
250 years  Sestercentennial  To express 2+1⁄2 in Latin it would be expressed as "halfthree". The term relates to being halfway [from the second] to the third integer. In Latin this is "Sestertius", which is a contraction of semis (halfway) tertius (third)—hence Sestercentennial.  
Semiquincentennial  Probably^{[attribution needed]} a modern coined term: semi (half) × quin (5) × centennial (100 years) = 250 years. Used by Brown University in 2015.  
Bicenquinquagenary  Used by Princeton University in 1996, Reading, Pennsylvania in 1998, and Washington and Lee University in 1999. It is a coined word for an anniversary of 250 years: bi (2) × cen(t) (100) + quinquagenary (50 years).  
Quartermillennial  
275 years  Bicenterquasquigenary  
300 years  Tercentennial  
Tercentenary  
Tricentennial  
Tricentenary  
350 years  Sesquarcentennial  Sesquarcentennial is a modern coined term; sesquarcentennial for 350 years is deduced here from the "Sestertius" definition for 250 years above. For 350 years it relates to being halfway from the third to the fourth integer; thus a contraction of semis (halfway) and quartus (fourth); hence Sesquarcentennial. Semiseptcennial is probably^{[attribution needed]} a modern coined term: semi (half) × sept (7) × cen(t) (100) × centennial (350 years).  
Semiseptcentennial  
375 years  Terquasquicentennial  
400 years  Quadricentennial  
Quadricentenary  
Quatercentenary  
450 years  Sesquincentennial  
500 years  Quincentenary  
Quincentennial  
600 years  Sexacentennial  
Sexcentenary  
700 years  Septcentennial  Probably^{[attribution needed]} a coined term; earliest known use in March 1988. Chiang Mai Septcentennial Stadium (Chiang Mai, Thailand) was completed in 1991.  
Septuacentennial  
800 years  Octocentennial  
Octocentenary  
900 years  Nonacentennial  
1000 years  Millennial  
1500 years  Sesquimillennial  Term broken down as sesqui (one and a half) millennial (1000 years)  
2000 years  Bimillennial  
3000 years  Trimillennial  
4000 years  Quadrimillennial  
5000 years  Quinmillennial  
6000 years  Sexmillennial  
7000 years  Septmillennial  
8000 years  Octomillennial  
9000 years  Novamillennial  
10,000 years  Decamillennial  
100,000 years  Centamillennial 
Symbols
Many anniversaries have special names. Etiquette in Society, in Business, in Politics and at Home by Emily Post, published in 1922, contained suggestions for wedding anniversary gifts for 1, 5, 10, 15, 20, 25, 50, and 75 years. Wedding anniversary gift suggestions for other years were added in later editions and publications; they now comprise what is referred to as the "traditional" list. Generally speaking, the longer the period, the more precious or durable the material associated with it.
There are variations according to some national traditions. There exist numerous partially overlapping, partially contradictory lists of anniversary gifts (such as wedding stones), separate from the "traditional" names. The concepts of a person's birthday stone and zodiac stone, by contrast, are fixed for life according to the day of the week, month, or astrological sign corresponding to the recipient's birthday.