I've heard people using あした (the common one) and あす (rather uncommon) to refer to "tomorrow".
I was wondering does anyone actually uses the reading みょうにち ?
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I have heard it used in formal (e.g. business) contexts. I have never heard it used in casual conversations among friends or family.
This is what you might expect, because Sino-Japanese words like みょうにち do tend to have a more formal feel than native Japanese words like あした or あす, when they exist alongside each other with similar meanings.