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!
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.