The UCG article Good Friday + Easter Sunday: It Doesn’t Add Up!, and other articles in that issue of the magazine, explain why they believe the Crucifixion was on a Wednesday afternoon in AD 31.
It's not as legible as I'd like, but the UCG produced this chart that shows the events aligned with both Roman and Hebrew days:
But then the explanation paragraph in that page confused me.
the Passover meal was eaten on Tuesday night and Wednesday sundown marked the beginning of the "high day"
Referring to "the Last Supper" as "the Passover meal" is definitely the cause of the confusion here.
It doesn't make sense for them to have eaten the Passover meal before it was slaughtered:
The animal was slain on the eve of the Passover, on the afternoon of the 14th of Nisan, after the Tamid sacrifice had been slaughtered, i.e., at three o'clock, …
— Passover sacrifice - Wikipedia
The UCG definitely teaches that Jesus died in the late afternoon on Wednesday, the day of preparation for the Passover, at the same time as the lambs were slaughtered.
This time is very significant, as they believe that the Passover Holiday was instituted to foreshadow the Crucifixion.
As to why that article referred to the supper as the Passover meal, that's a good question.
The various COG organizations do celebrate Passover at the beginning (early night) of the 14th (including a footwashing ceremony and their once-a-year bread and wine "communion" ceremony).
The Passover sacrifice was killed and eaten as a simple supper after sunset just after the 14th begins (only the meal, not the day itself is considered holy).
24 hours later, the first day of the Feast of Unleavened Bread (a high holiday) is celebrated for a week.
This holy feast beginning on the 15th became much larger following the return from Babylonian captivity:
And the children of Israel that were present at Jerusalem kept the feast of unleavened bread seven days with great gladness …
And the whole assembly took counsel to keep other seven days: and they kept other seven days with gladness.
For Hezekiah king of Judah did give to the congregation a thousand bullocks and seven thousand sheep; and the princes gave to the congregation a thousand bullocks and ten thousand sheep … .
2 Chronicles 30:21,23,24.
The supper, the feast on the first day, and the entire festival can all be referred to as "the Passover", as can be the sacrificed meat.
So yes, the "last supper" was a Passover meal, but it was the simple ceremonial one, not the large feast(s) of the following days.
The New Testament Passover, Doctrinal Study Paper provides much much more detail on this topic.
In addition to the above chart, the same issue of the magazine contains this interesting information,
indicating that the idea of the Wednesday Crucifixion and Saturday Resurrection, with a Tuesday/Wednesday night Passover supper were common in the early days of the Church,
though eventually the current Friday/Saturday view of mainstream Christianity prevailed.
Centuries-Old Documents Show Evidence for a Wednesday Crucifixion
Did you know there is additional historical evidence for a Wednesday crucifixion?
Although it was a minority position and ran against the prevailing teachings of the Roman church,
some early historical documents acknowledge a Tuesday night Passover, a Wednesday crucifixion
and a Saturday afternoon resurrection—matching the biblical record.
Around the year 200, a document purporting to pass on apostolic instruction,
called the Didascalia Apostolorum,
mentions that the last Passover of Jesus Christ and His disciples was on a Tuesday night.
This document states: “For when we had eaten the Passover on the third day of the week at even
[Tuesday evening], we went forth to the Mount of Olives; and in the night they seized our Lord
Jesus. And the next day, which was the fourth of the week [Wednesday], He remained in ward
in the house of Caiaphas the high priest” (emphasis added throughout).
Paradoxically, the text goes on to mention that Jesus was crucified on a Friday—showing
great confusion about the dates, for the biblical account clearly states that Christ was
crucified in the daylight period following the night of that Passover meal and arrest.
Nonetheless, the document demonstrates that Passover was then understood by some
to have been on Tuesday evening, which would place the crucifixion on the next day, Wednesday.
Epiphanius (A.D. 367-403), the bishop at Salamis, wrote that “Wednesday and Friday are
days of fasting up to the ninth hour because, as Wednesday began the Lord was arrested and
on Friday he was crucified.”
As we can see, even though the prevailing view was that Friday was the day of the crucifixion,
Wednesday was known as the day of Christ’s arrest
(happening as it did in the early predawn hours that day).
By the fifth century, Easter Sunday celebrations were widespread.
However, a church historian of the time named Socrates notes in a section of his history
titled “Differences of Usage in Regard to Easter” that some Christians celebrated
the resurrection on the Sabbath rather than on Sunday.
As he put it, “Others in the East kept that feast on the Sabbath indeed.”
Bishop Gregory of Tours (538-594), although himself believing in a Sunday resurrection,
noted that many believed Jesus rose on the seventh day of the week,
stating, “In our belief the resurrection of the Lord was on the first day,
and not on the seventh as many deem.”
So rather than a monolithic acceptance of the Good Friday–Easter Sunday scenario,
there was confusion about the timing of Christ’s crucifixion in early centuries.
Moreover, these historical records show that a minority of Christians during
that time understood the biblical timing of a Tuesday evening Passover,
a Wednesday crucifixion and a late Saturday afternoon resurrection.
— Mario Seiglie
— March–April Beyond Today Magazine
