“I wish it were/was/had been Friday.”

[sitifan]

“I wish it was Friday.” means that the speaker is asking that something that is now scheduled for Thursday, should instead happen a day later, on Friday. So the subliminal meaning of the sentence is “I wish it [the event that is scheduled for Thursday, instead] was [scheduled for] Friday.” It is also possible that the simple past tense (“was”) could be replaced with the past perfect (“had been”) to make the difference more obvious. (My bold.)
Source: https://www.quora.com/Which-one-is-...I-wish-it-were-Friday/answer/Frank-Dauenhauer

Is it really possible that that the simple past tense (“was”) could be replaced with the past perfect (“had been”) here?

 

[emsr2d2]

“I wish it was Friday no full stop here” means that the speaker is asking that something that is now scheduled for Thursday, should could instead happen a day later, on Friday.

It can mean that but most people would take it to mean "I wish today was Friday [but it's a different day]". For it to mean what you said, the preceding context would need to make that clear. We don't have any context.

So the subliminal meaning of the sentence is “I wish it [the event that is scheduled for Thursday, instead] was [scheduled for] Friday.” It is also possible that the simple past tense (“was”) could be replaced with the past perfect (“had been”) to make the difference more obvious. (My bold.)

It's not subliminal. It would require the listener to be a mind-reader!

Source: https://www.quora.com/Which-one-is-...I-wish-it-were-Friday/answer/Frank-Dauenhauer

Is it really possible that that the simple past tense (“was”) could be replaced with the past perfect (“had been”) here?

I can't open the link but I'm going to say that if someone meant "I wish the party were on Friday instead of Saturday", they couldn't use "I wish the party had been on Friday instead of Saturday" instead.

"I wish the party were on Friday instead of Saturday" refers to a party in the future. Note that the majority of BrE speakers these days would use "was", not "were", there. The use of the subjunctive isn't as common as it used to be.
"I wish the party had been on Friday instead of Saturday" refers to a party that happened on Saturday but the speaker would have preferred a Friday party.

 

[sitifan]

I can't open the link ...

Which one is correct “I wish it was Friday.” or “I wish it were Friday.”?

This is a tricky question. Both are correct, but under different conditions.

“I wish it were Friday.” means that the speaker is asking for something that does not now exist and cannot be made to happen. For example, suppose that today is Thursday, and the speaker wants it to be Friday—which is not possible for anyone to cause to happen. In this case, the sentence is in the subjunctive mood, a mood that shows something is wished for but is not possible.

“I wish it was Friday.” means that the speaker is asking that something that is now scheduled for Thursday, should instead happen a day later, on Friday. So the subliminal meaning of the sentence is “I wish it [the event that is scheduled for Thursday, instead] was [scheduled for] Friday.” It is also possible that the simple past tense (“was”) could be replaced with the past perfect (“had been”) to make the difference more obvious.

 

[Tarheel]

"I wish it were Friday" means the speaker is antsy for Friday to come. He is looking forward to something that's going to happen that day, and he wants to come sooner.

"I wish it was Friday" means the speaker wishes that the eagerly anticipated day (Friday) was the current day (Friday) and that he didn't have to continue waiting or at least not much longer.

 

[emsr2d2]

I realised there's something I forgot to add in my original response. The simple addition of "on" would make the original sentence much clearer (in the context you described).

Bob: The meeting is on Saturday.
Fred: I wish it were/was on Friday.

You'd hear both "were" and "was" in Fred's response from BrE speakers. Older speakers are more likely to use the subjunctive "were".