"Birthday" vs. "anniversary"
Are there general guidelines for using "anniversary" vs. "birthday"?
E.g., birthdays are generally for... well, birthdays. It's also used for some notable historical dates regarding countries ("Our nation's birthday") and cities.
Is there a meaningful, generally-accepted differentiation?
Birthday:
- using the stricter definition, a birthday is the anniversary of one's birth.
Anniversary:
is a word used to commemorate special occasions, like weddings, or first dates ( important job promotions for instance) or important purchases ( a house) and things like that. Interestingly enough, even though we don't usually refer to birth dates as "anniversaries" ( unless we are referring to famous people) we do use the word when referring to death dates.
The 100th anniversary of the birth of composer Benjamin Britten.
Taoiseach commemorates 90th anniversary of Michael Collins’ death
Anniversary is for things that are celebrated annually. And this includes birthdays, of course, but a birthday is a special case of anniversary, and you would almost never say birth anniversary, but almost always birthday.
However, unless you qualify the anniversary, it is generally taken to mean a "wedding anniversary". If I say, "Today is my anniversary," unless the context clearly indicates otherwise, I would be speaking of my wedding anniversary.
So, for other anniversaries, use a qualifier. For my employment I would possibly say "work anniversary" or something similar.
For notorious anniversaries, such as a nation's birthday, we seem to usually use a recognized proper name. For the United States, July 4 is referred to as "Independence Day." In France an important anniversary is "Bastille Day". In New Zealand, "Dominion Day."
Birthday is generally used for people's birth dates and sometimes for a country's, as you've said. Anniversary, on the other hand, is generally used for a marriage or relationship between two people, for example the day of the year two people got married. It is also used to signify the day an important event happened. For example, November 9th of this year will be the 25th anniversary of the Berlin Wall coming down.