“I was very happy yesterday” 是表达这个意思的常用说法。
"I was" 是过去时，表示昨天。"very" 是副词，表示非常，"happy" 是形容词，表示高兴。
例句：
I was very happy yesterday when I received my promotion. (昨天我收到晋升的消息时非常高兴)
Yesterday was a great day, I was very happy. (昨天是个好日子，我非常高兴)
I had a lot of fun yesterday, I was very happy. (昨天我玩得很开心，非常高兴)