Noun
たんじょうび
「君の誕生日会に友達を連れて行ってもいい？」「もちろん！ 人数が多ければ多いほど楽しいしね。」
"Can I bring a friend to your birthday party?" "Of course! The more people we have, the more fun it will be."
Remember how 誕生 is birth? If you add day on there, you have a birthday. Happy birthday!
This is a jukugo word, which usually means on'yomi readings from the kanji, but with this word 日 uses the kun'yomi ひ, which is turned into び due to rendaku.