Yes, lying is bad.*

Human communication is a small miracle by which thoughts in one mind are reproduced in the mind of another. Watch this: "The Arctic Tern flies from its Arctic breeding grounds to the Antarctic and back again each year." citation You were just looking at some marks on a screen and now you have thoughts of a bird.

Putting thoughts which you do not know to be true in someone else's mind is irresponsible and unkind. Before we learn that adults are not to be trusted, as children we are crammed full of ideas that we only later learn were false. Giving confident bad directions is worse than giving no directions.

Putting thoughts which you know to be untrue in someone else's mind is malicious. You are causing a person's world view to be less accurate.

That said, behavior occurs amid a sea of competing values, and we make choices as best we can. Making the birthday party a pleasant surprise may be more important to us than being honest about our plans.

*To be honest, I'm not really sure. It's a good question.