こんにちは。
様々な言い方ができると思いますが、例えば下記のような表現はいかがでしょうか：
・if I had known that today was his birthday
今日が彼の誕生日だと知っていたら
if only I had known のように only を加えると「誕生日だとさえ知っていたら」のように少しだけニュアンスを変えることもできます。
例：
If I had known today was his birthday I would've got him a present.
今日が彼の誕生日だと知っていたらプレゼントを買ってきたのに。
If only I had known today was his birthday...
今日が彼の誕生日だということさえ知っていれば…
ぜひ参考にしてください。
If I had known it was his birthday today, I would have brought a gift.
- *If I had known it was his birthday today, I would have brought a gift.*
- *I wish I had known it was his birthday today.*
また、少し感情を込めたい場合には
- *I wish I had known...*（知ってたらよかったのに）
という言い方もよく使われます。
（何か持ってきたのに）
アメリカ合衆国 
