Three days and three nights - It will be seen in the account of the resurrection of Christ that he was in the grave but two nights and a part of three days. See Matthew 18:6. This computation is, however, strictly in accordance with the Jewish mode of reckoning. If it had "not" been, the Jews would have understood it, and would have charged our Saviour as being a false prophet, for it was well known to them that he had spoken this prophecy, Matthew 27:63. Such a charge, however, was never made; and it is plain, therefore, that what was "meant" by the prediction was accomplished. It was a maxim, also, among the Jews, in computing time, that a part of a day was to be received as the whole. Many instances of this kind occur in both sacred and profane history. See 2 Chronicles 10:5, 2 Chronicles 10:12; Genesis 42:17-18. Compare Esther 4:16 with Esther 5:1.
It was a NT prediction by Jesus that just said three days. Any part of the day counts..
day one - Friday
day two - Sabbath
day three - Sunday
good study about that here..