Which is correct, "a celebration is planned by the government" or "a celebration has been planned by the government"?
Why do you think one of them might be "incorrect"?
They are both grammatical, but have different structures, and hence slightly different meanings, though in practice they're probably interchangeable in most circumstances.
In is planned, "planned" is an adjective. The sentence is describing the state of the celebration.
Has been planned is a verbal expression, a "present perfect", so talking about an activity and its result.
To my ear, "has been planned" puts a little more focus on the activity of planning that has happened, so suggests that the government has put some work into the planning.
"Is planned" does not deny that possibility, but it doesn't focus on the activity. It might suggest that less work has gone into the planning up to now. But on the other hand, it might just mean that the speaker is more interested in the celebration than in the planning.