"It was a long day, and I am very tired."
Translation:Это был долгий день, и я очень устал.
I believe not using «э́то» would change meaning, but I’m not sure I can describe the meaning exactly.
For me, «Был до́лгий день» sounds as if you’re introducing a situation, e.g. if this is a first sentence in a story, and this is not the main point you’re going to make. «Э́то был до́лгий день» sound OK in other situations too, not just as a beginning of the story.
I'm not sure my understanding is 100% correct because it's not easy to expain the difference.
I guess tha "это" indicates here a specific day , on which the speaker is referring to. The one which has just past. Not another one.
My question is the same, is it wrong to say just ''был долгий день'' For me it sounds right, and I know I definitely heard people say such constructions, but I would like to know the opinion of a native Russian speaker.