what day yesterday is vs what day yesterday was

Some examples from the web:

• Oct 2, 2010 ... Super Hint: Hints Two and Three almost give the answer away. Since yesterday's day after tomorrow is Sunday, yesterday is Friday. Tomorrow's day before yesterday is yesterday. Do you remember what day yesterday is?
• Dec 28, 2010 ... "I am so tired I don't even know what day yesterday is," she said. Cathay Pacific spokesman Gus Whitcomb said the planes had taken off under the assumption that they would have somewhere to go upon landing. U.S. airlines ...
• Dec 28, 2010 ... "I am so tired I don't even know what day yesterday is," she said. Steve Coleman, a spokesman for the Port Authority of New York and New Jersey, the airport operator, said airlines were dispatching planes to the airport ...
• Dec 28, 2010 ... "I am so tired I don't even know what day yesterday is," she said. Cathay Pacific spokesman Gus Whitcomb said the planes had taken off under the assumption that they would have somewhere to go upon landing. U.S. airlines ...

Some examples from the web:

• Aug 31, 2014 ... Do You know What Day Yesterday Was? What About Today and Tomorrow? Learn Days of the Week with This Great Video. “Yesterday, Today, Tomorrow” ( Level 2 English Lesson 05) CLIP - Days in English, Learn English.
• ... Sunday" the activity should be continued the rest of the week by repeating the same word format "today is ______, yesterday was___________ ," versus the next day saying "What is today, does anyone remember what day yesterday was ?".
• How can a child tell what day yesterday was if s/he does not yet know the days of the week? How can a child tell that art will end in ten minutes at 2:00 if s/he cannot yet tell time? These are skills that some children have to be expressly taught.
• Jul 9, 2014 ... ... we will go swimming." Provide as much hand-under hand assistance as necessary and remember to label spatial concepts as you explore, e.g., "Let's start at the top of the calendar to see what day yesterday was." ...