According to Calvinists, Luke 22:8 what date did it happen? Luke 22


  (7)Then came the day of Unleavened Bread on which the Passover lamb
  had to be sacrificed.  (8)Jesus sent Peter and John, saying, "Go
  and make preparations for us to eat the Passover."


From the info of verse 7, so when Jesus say "Go and make preparations for us to eat the Passover" I guess it happen in the time where the Jewish people either are preparing a lamb to be sacrificed (sacrificed in the same day) or even maybe some of the Jews are already sacrificing the lamb when Jesus say that. 

Assuming my guessing is correct, the question is :
What date that event happen ? 
13 Nissan ? 14 Nissan ? or 15 Nissan ?

The Catholic opinion is sought here.

The Lutheran opinion is sought here.
The Last Supper seems in the Synoptic Gospels to take place at the time of the Passover Meal. This would imply that Luke 22: 7 took place in the morning of Nissan 14.

However, John's Gospel appears to put the Crucifixion on Nissan 14.

In his commentary Calvin posed the question this way:


  How did Christ observe that ceremony on the day before the whole nation celebrated the public Passover? For John plainly affirms that the day on which Christ was crucified was, among the Jews, the preparation, not of the Sabbath, but of the Passover,


In Calvin's view then, Jesus and His disciples observed the Passover a day earlier than most of the Jews.  

It may be worth mentioning that the Jewish calendar was used for religious feasts but the Roman civil calendar was in more general use. The month of Nissan, like all months, began with the first sighting of the New Moon. However if the barley was not sufficiently ripe at the start of the month then they would wait until the following New Moon to announce that Nissan had begun. 

In this way there were sometimes 13 months in the year and sometimes 12, and it would not be known until just before the month started whether it would be Nissan or not. Since the fourth century AD Jews have used a mathematical formula to add a 13th month 7 times every 19 years, but at the time of Christ it was done on an ad hoc basis.

Back to Calvin, how does he explain why Jesus celebrated Passover a day earlier than most of the Jews? He says:


  I have no doubt, therefore, that Christ observed the day appointed by the Law, and that the Jews followed a custom which had been long in use. First, it is beyond a doubt that Christ was put to death on the day before the Sabbath; for he was hastily buried before sunset in a sepulchre which was at hand, (John 19:42,) because it was necessary to abstain from work after the commencement of the evening. Now it is universally admitted that, by an ancient custom, when the Passover and other festivals happened on Friday, they were delayed till the following day, because the people would have reckoned it hard to abstain from work on two successive days.


Calvin's view was that there was a Jewish tradition that if Passover fell on a Friday then it was transferred to Saturday instead. This is because Saturday was the Sabbath and people thought it too hard to abstain from work two days running. Jesus however celebrated it at the correct time according to the Law.

If this is true then the incident in Luke 22: 7 took place on Nissan 14.

This was Calvin's view, but is it the Calvinist view? Not necessarily, there Is no definite doctrine on the matter. 

The Reformed view generally would usually be that the apparent contradiction between Luke and John must have some explanation. Rather than choose between conflicting accounts the Reformed tradition would tend to find some way to choose both, and if none can be found then that still doesn't prove there isn't one. 

Beyond this, is there anything in Calvin's theory which is representative of his thought, beyond being his personal view on a non-dogmatic issue? Maybe.

Calvin's idea is that the priests had been doing it all wrong for a very long time. This is a motif seldom far from Calvin's thought. As in Rome at Calvin's time, so in Jerusalem at Christ's.

Secondly Calvin says the idea of having to abstain from work for two days running would be too hard to bear. The modern weekend did not exist in sixteenth century Geneva. Perhaps Calvin's aversion to two days off is an an example of the Calvinist Work Ethic.   

Nevertheless, we cannot really say there is a Reformed position on this.