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. 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.
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. The important dates in a sitting monarch's reign may also be commemorated, an event often referred to as a "Jubilee".
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.
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 described with Latin words or Roman numerals.
Latin terms for anniversaries are mostly straightforward, particularly those relating to the first twenty years (1–20), those relating to multiples of ten years (30, 40, 60, 70 etc.), and those relating to multiples of centuries or millennia (100, 200, 300, 1000, 2000, 3000, etc.) In these instances, the anniversary generally comes from a derivative of the Latin word for the respective number of years. However, when anniversaries relate to fractions of centuries (125, 150, 175, 250 years—i.e., 1.25, 1.5, 1.75, and 2.5 centuries), the situation is not as simple.
Roman fractions were based on a duodecimal system. From 1⁄12 to 8⁄12 they were described as multiples of twelfths (uncia "twelfth"; the source of the English words inch and ounce) and from 9⁄12 to 11⁄12 they were described 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 three-quarters (dodrans). Dodrans is a Latin contraction of de-quadrans 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 literally for 1½. Whereas "secundus" is Latin for "second", or "bis" for "twice" these terms are not used such as in sesqui-secundus. Instead just sesqui (or ses) is used by itself. This may be because it relates to a ratio of one—thus it means "and a half" compared to "one". (It may also be because 1⁄8 in Latin is sescuncia, sescunciae (from sesqui- + uncia, i.e., 1½ uncias) defined as one-and-a-half twelfths ( 3⁄2 × 1⁄12 = 1⁄8).)
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. See wedding anniversary for a general list of the wedding anniversary symbols; however, there are variations in some national traditions.
Furthermore, 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.