noun
half birthday
A celebration held exactly six months before or after one's actual birthday, popular for babies and sometimes for adults.
赤ちゃんのハーフバースデーに小さなケーキを用意した。
We prepared a small cake for the baby's half birthday.
友達とハーフバースデーパーティーを開いた。
I threw a half birthday party with my friends.