"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.
Это is more accurately translated to "it" in English: "It was a long day" is a far more usual expression.
"That was a long day...." suggests an emphasis on the particular day which is the subject of the sentence, in a way which makes "that day" more significant than other days, which were mostly shorter.
"It was a long day" on the other hand simply observes that the day was long, no different from any other day.
You can also get a cyrillic keyboard on your phone. I use GBoard (google) which switches easily between languages. On my computer I use TypeIt for a phonetic keyboard +(which means I can touchtype using the keys I am used to for most letters that have similar sounds) or I use the Windows 10 language pack which gives you the option of a virtual keyboard or using your standard keyboard with a normal Russian keyboard layout. You can put stickers on your keyboard to remind you where the Cyrillic letters are.