•
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 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 to they were expressed as multiples of twelfths (
uncia "twelfth"; the source of the English words
inch and
ounce) and from to they were expressed as multiple twelfths less than the next whole unit—i.e. a whole unit less , or 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 precisely for . While
secundus is Latin for "second", and
bis for "twice", these terms are not used such as in sesqui-secundus. Instead
sesqui (or
ses) is used by itself. ==Symbols==