answer:I believe that using “were” would be more appropriate in situations where the event did not happen, or is simply just a possible event, whereas “was” is technically reserved as the 1st or 3rd person singular past tense of the verb “to be.” However, language is always changing, and speaking as a native English speaker in America, saying “if the interview was on Monday” would be 100% understood by another native English speaker in America. In this case one could argue that both are grammatically correct. Technically speaking though, you would use “if the interview were on Monday” and “was” in the case of “The interview was on Monday.”