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.
"Can I bring a friend to your birthday party?" "Of course! The more people we have, the more fun it will be."