Why would "What are we doing here in the evening?" be considered wrong? I will report this as an error.
увечері means "in the evening" уночі means "at night."
And "вечір" here is when it's dark outside, "ніч" is when most people sleep
Why is it at night without 'the' and in the evening with 'the'? As a native Slav I have problem with that.
AFAIK there aren't any rules here. Take it as it is:)