If yesterday is tomorrow and tomorrow is today then what is yesterday if today would be tomorrow?

If today is tomorrow, then yesterday's tomorrow is today.

Another answer: Yesterday's tomorrow is today. This is a fact. This is reality.

But if what we think is today is really tommorow, then we are a day behind! So rather than yesterday's tomorrow being today, 'yesterday's tomorrow' must be one more day than today, i.e. it must be tomorrow!