This is actually a grammar and a maths puzzle. the statement 'If yesterday was tomorrow' can be interpreted two ways:
a: If tomorrow were to be replaced by yesterday
b: If yesterday had been the day it will be tomorrow
The second option seems more likely because the phrase 'If Yesterday was tomorrow' uses the past tense 'was'. Grammatically, the first option would require the question to be 'If yesterday were to replace tomorrow', or something similar.
If we interpret it the first (grammatically incorrect) way, then:
Yesterday = t-1
Today = t
Tomorrow = t+1
If t+1 ==> t-1
Then t ==> t-2
t-2 = Friday
t = Sunday
If we interpret it the second way, then:
If t-1 ==> t+1
Then t ==> t+2
t+2 = Friday
t = Wednesday
The answer is therefore Wednesday