Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: When you celebrate something, you have to yell out your celebrations. Yell out いわう three times! It's kind of like a combination of the word いい (good) and "wow." いいwow! いいwow! Just make sure the いい gets shortened, because it's getting combined.
We are going to a Chinese restaurant to celebrate grandpa’s 60th birthday on Sunday.