What is a day in the Torah?

One Biblical day spreads over two calendar Roman days from evening to evening.

When reading the Bible, we must take into account that a Biblical day is not the same like the same day reckoning in other Calendars. A Biblical day falls between two evenings: Friday evening to Saturday evening is Saturday in Hebrew.

And God called the light, Day. And He called the darkness, Night. And the evening and the morning were the first day. Genesis 1:5 MKJV

When we read measurements of time in the Bible, we must keep in mind that the time involved is Biblical time which is different from other time keeping because a biblical day starts in the evening before darkness, which is night (Genesis 1:5). In considering the timing of some events in the Bible, biblical pundits argue that the Jews count part of a day as a day. But we see in Exodus 12:18-19 that the day is well defined in time: from the fourteenth to the twenty-first is 7 days not 8 days.

"In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even. Exodus 12:18 ASV

Seven days shall ye eat unleavened bread; even the first day ye shall put away leaven out of your houses: for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from Israel. Exodus 12:15 ASV"

So, there is no such thing as inclusive dating that would have made the length of the unleavened bread 8 days. The Bible subtracts the dates: 21-14=7. Period. Exodus 12:18-19. The biblical day covers 2 Roman secular days like the 7th day Sabbath which covers Friday and Saturday. In other words, a biblical day falls between 2 evenings.

Therefore, during the Passover week, the length of 3 days cover the span of 4 named days. For example, Friday, Saturday, and Sunday do not count 3 days, from the point of view of Exodus 12:18-19. Friday, Saturday, Sunday, and Monday do cover the length of 3 days from the perspective of Exodus 12:18-19. Remember: 'There was an evening, and there was a morning'. Genesis 1:5.

Jesus was mindful of Exodus 12:18-19, when He invoked the experience of Prophet Jonah in the belly of the fish for 3 days and 3 nights, in the boundary of His prophesied death and resurrection timing of '3 days', 'third day', 'after the third day'.

The arithmetic of Friday, Saturday, early Sunday boundary for the timing of the death and resurrection of Jesus fails the tests of the Prophet Jonah and of Exodus 12:18-19. One night is missing: Sunday night. In other words, if Jesus died at the ninth hour, the resurrection must take place on Monday. Counting part of a day to squeeze the resurrection for Sunday falls afoul of the definition of 7 days (also of a day) of Exodus 12:18-19, as the Jews were still celebrating the 7 day long ordinance.

Fortunately, some correct translations of the related texts in the New Testament place the resurrection on a Saturday which is specified as the first of the seven sabbaths that lead to Pentecostal (fiftieth) day, which has always been a Sunday.

Matthew 28:1 LITV But late in [the] sabbaths, at the dawning into [the] first of [the] sabbaths, Mary the Magdalene and the other Mary came to gaze upon the grave.

Note that [ ] means that the word does not appear in the original text but is added by the translators.

Again, one biblical day covers 2 nominal days in the Julian or Gregorian calendars.