Happy Birthday
expression
誕生日 is birthday. おめでとう is like "congratulations." So, congratulations for your birthday is like happy birthday!
The reading is the same as 誕生日, so if you know that you know this!
弟はだれからもお誕生日おめでとうって言ってもらえなかったと言って、すねていましたよ。
My little brother was sulking because nobody said happy birthday to him.
歌詞が「お誕生日おめでとう」しかない歌なのに、バズって売れた。
A song with just the lyrics from お誕生日おめでとう(Happy Birthday) managed to go viral and become a hit.
遅れたとしても、「お誕生日おめでとう」って言う方が言わないよりもいいよ。
Better late than never with saying happy birthday.