an anniversary of the day on which a person was born (or the celebration of it)
"We are planning a big party for my birthday next week to celebrate another year around the sun."
The anniversary of the day on which someone is born.
"We celebrated my birthday last week by having a large dinner party at home."
In plain English: A birthday is the day each year when you were born and people usually celebrate with gifts and cake.
"We are planning a big party for my birthday next week."
Usage: A birthday refers to the specific annual date marking the anniversary of your birth, not the event or party celebrating it. Use this term when identifying the calendar day itself rather than the festivities surrounding it.
To celebrate one's birthday.
"We are planning a big party to celebrate our son's birthday next week."
In plain English: You cannot use the word birthday as a verb because it is only a noun that refers to the day you were born.
"She decided to birthday her friend with a surprise party."
Usage: Birthday" is not used as a verb in standard English; you must use the verb "celebrate" to describe the act of marking your birth date. Using "birthday" as an action is incorrect and should be avoided in all everyday writing and speech."
The word "birthday" comes from Old English ġebyrddæġ, which literally means "birth day." It combines the words for birth and day to describe the anniversary of someone's birth.