Consider the logical statement: "If yesterday were tomorrow, then today would be Friday." Assuming this statement is true, what actual day of the week is today?

Introduction:
This is a classic logical and calendar-based puzzle. The sentence "If yesterday were tomorrow, then today would be Friday" is a hypothetical (counterfactual) statement. Our goal is to translate this sentence carefully into relationships between days of the week and then work out what day "today" must actually be.

Given Data / Assumptions:
The statement "If yesterday were tomorrow, then today would be Friday" is assumed to be true. We use the usual meaning of yesterday (one day before today), today, and tomorrow (one day after today). We must find the actual day corresponding to "today".

Concept / Approach:
The key idea is that between "yesterday" and "tomorrow" there is a gap of 2 days. If we imagine a hypothetical situation in which what is now "yesterday" becomes "tomorrow", we effectively move our frame of reference back by 2 days. In that hypothetical world, the word "today" refers to the day that is 2 days earlier than the real today. The statement then says that in that hypothetical world, "today" is Friday. From that information we can work backward to the real day.

Step-by-Step Solution:
Step 1: Let the real today be T. Then real yesterday = T − 1 day, and real tomorrow = T + 1 day. Step 2: Interpret "If yesterday were tomorrow". For yesterday to be tomorrow, we must shift our point of view 2 days earlier. From that earlier viewpoint, call it T′, yesterday (relative to T′) is T′ − 1 and tomorrow is T′ + 1. We want the real "yesterday" (T − 1) to play the role of "tomorrow" in the hypothetical, so: T − 1 = T′ + 1 ⇒ T′ = T − 2. Thus, the hypothetical "today" is T′ = T − 2. Step 3: Use the part "then today would be Friday". In that hypothetical scenario, "today" is T′, and it is said to be Friday. So T′ is Friday ⇒ T − 2 is Friday. Therefore, the real today T is 2 days after Friday. 2 days after Friday is Sunday.

Verification / Alternative check:
If today is Sunday, then yesterday is Saturday and tomorrow is Monday. Now imagine a strange world where yesterday (Saturday) were instead tomorrow. That means we shift our reference back by two days, to Friday as "today". In that world the statement "today would be Friday" is indeed true, so Sunday is consistent with the puzzle.

Why Other Options Are Wrong:
Saturday or Monday: If you run through the same two-day shift logic, these do not make the hypothetical "today" equal to Friday. Tuesday or Wednesday: These are even further away and cannot satisfy the described conditional statement.

Common Pitfalls:
People often try to treat the sentence as a simple direct equation between days, without recognising it as a hypothetical shift of the time frame. The crucial insight is that "yesterday were tomorrow" implies moving two days backward in time to define the hypothetical "today".

Final Answer:
The only day that makes the statement true is Sunday.