It is so thoughtful of you to ask me, but unfortunately I have a rather important previous engagement on that day.

您对我的邀请真是想得周到。但不巧的是，那一天我预先已有重要的约会。

youdao

I'd love to, but I'm afraid I have a previous engagement.

我很宁愿答应往，但生怕已跟他人事前有约。

youdao

I regret to say (that) I have a previous engagement that evening. However, thank you FOR thinking of me.

很遗憾，那天晚上我另有约会。不过，我还是感谢您想到了我。

youdao