[header] How to say happy birthday in japanese [title] Tell friends " tanjoubi omedetou. [step] " this is a casual, informal way of telling someone " happy birthday. " [substeps] Only use this expression with people you are familiar with and those whom you are able to speak informally. 
Generally speaking, this includes friends, most classmates, most children, and most siblings or younger cousins. Avoid using this with anyone who has a higher status than you, such as a teacher, supervisor, stranger, or elder.