What is a day in the Torah?

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
John 11:9 ASV
(9) Jesus answered, Are there not twelve hours in the day? If a man walk in the day, he stumbleth not, because he seeth the light of this world.

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.

Evening is not sunset, nightfall, or night. Sunset marks the entering of the sun. Nightfall is the complete disappearance of the sun at the end of the evening. Night is the total darkness.

The day starts at a specific time, as the Jews of antiquity started preparing it at the ninth hour, according to the witness of Josephus. The ninth hour cannot logically be a moving time which depends on the time of the moving sunset.

There is the evening, there is the night, there is the morning, there is noon or midday (Genesis 43:16), and obviously there is the afternoon or the going of the sun (Exodus 17:12, Joshua 1:4, Psalms 50:1, 113:3; Deuteronomy 11:30).

The Bible does not support the evening to coincide with the entering (Strong's H3996) of the sun in the horizon, which is sunset (Exodus 17:12, Joshua 1:4, Psalms 50:1, 113:3). Rather, the evening coincides with the going of the sun, which is not entering.

In the translations, the word 'down' in the expression 'going down' to mean 'sunset' is an addition to the meaning of the Strong's Hebrew H935. That is, it does not exist in relation to the going of the sun.

Biblically, evening is not sunset, nightfall, or night.

Evening Clock Time in the Parable of the Vineyard: the 12th Jewish hour which is 3 PM in our Roman Standard Time.

Matthew 20:8-12 ASV
(8)  And when even was come, the lord of the vineyard saith unto his steward, Call the laborers, and pay them their hire, beginning from the last unto the first.
(9)  And when they came that were hired about the eleventh hour, they received every man a shilling.
(10)  And when the first came, they supposed that they would receive more; and they likewise received every man a shilling.
(11)  And when they received it, they murmured against the householder,
(12)  saying, These last have spent but one hour, and thou hast made them equal unto us, who have borne the burden of the day and the scorching heat.

Since Jesus says that there are 12 hours in a day, do we start the Sabbath at the 12th evening hour or the first evening hour after the 12th hour? John 11:9.

I have chosen the first hour to let the 12 hours division of the day be complete but is it not more correct to start the new day at the 12th hour instead at the 1st hour?

Would Jesus refer to the 12th hour of the previous day as the first hour to start the next day? We can see the division of the biblical time from 12-4, 4-8, 8-12. That would be 3 watches.

We read of the morning watch in Exodus 14:24. We read of the middle watch in Judges 7:19. It implies that, if there is a middle watch, there must be 4 watches for each 12 hours. Could the divisions of morning, noon, evening, night be also watches?

Psalms 90:4 speaks of a watch in the night.

Jesus came unto the disciples in the fourth watch of the night walking on the sea, according to Matthew 14:25 and Mark 6:48. 

Matthew 24:43 speaks of not knowing in what watch the thief would come. Do thieves come during the day? If thieves were expected to break in at night, Matthew 24:43 implies that the night was divided into watches as Matthew 14:25 suggests.

Luke 12:38 speaks of the second and of the third watch. So, there were at least 4 watches.

So, a division of 12 hours into 4 slots is more likely so that the Sabbath day as an equal day also is divided into 4 watches: 1-3, 4-6, 7-9, 10-12.

Hence, it makes sense that the biblical Jewish Sabbath day starts at the first hour of the first watch.