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
In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening. Seven days there shall be no leaven found in your houses. Exodus 12:18-19 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, some 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.
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.
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. '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.
Besides, 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.
A biblical day covers 2 nominal days.