What is the difference between yesterday and one day ago?
Solution 1:
There is no real difference that would be recognized and respected by all writers and readers.
However, I can see some justification in separating the two. If it is 6 o’clock in the morning, then yesterday could refer to any time before you went to bed for the night during the previous day, including for example 6 at night.
On the other hand, a day ago would be more apt to refer to a time close to exactly 24 hours earlier, so 6 in the morning again.