birth·day 
/ˈbɚθˌdeɪ/ noun
plural birth·days /ˈbɚθˌdeɪ/ noun [count] 1 : the day when someone was born or the anniversary of that day
▪ Her birthday is September 30th. ▪ Today is his 21st birthday. ▪ Happy Birthday! — often used before another noun ▪ a birthday party/gift 2 : the day when something began
▪ The company just celebrated its 50th birthday.
▪ Her birthday is September 30th. ▪ Today is his 21st birthday. ▪ Happy Birthday! — often used before another noun ▪ a birthday party/gift
▪ The company just celebrated its 50th birthday.
birthday suit
informal + humorous ◊If you are wearing your birthday suit, you are naked.
▪ We were standing there in nothing but our birthday suits.
▪ We were standing there in nothing but our birthday suits.
