This can certainly be a puzzling question! By our way of counting, the third day would seem to be on Monday, but it seems the phrase "for three days" can really indicate parts of three days. He was in the grave for part of Friday, all of Saturday, and part of Sunday, so parts of three days. Let's look at a few other scriptures where the same concept is used. Genesis 42:17-18 says "he imprisoned them all for three days. On the third day, Joseph said to them, "Do as I say and you will live, for I fear God." This is an example that shows the way they counted time in biblical ages. In this case, they were imprisoned "for three days," but "on the third day," their sentence ended. Another example that shows this is in 2 Chronicles 10:5,12. "He said to them, "Go away for three days, then return to me." So the people went away . . . Jeroboam and all the people reported to Rehoboam on the third day, just as the king had ordered, when he said, "Return to me on the third day." Here, we have two accounts of the same conversation. One is recorded as "go away for three days" while the other says "return to me on the third day." From this example, we can clearly see that the two phrases meant the same thing.
Now, let's look at Jesus' specific case. We know that he died on Friday because it was the day before the Sabbath which was Saturday (see Mark 15:42-46, Luke 23:50-54, John 19:14,31,42). And we also know that he was raised on the first day of the week, which would be Sunday (see Mark 16:9). Luke 24:21 reads, "But we had hoped that he was the one who was going to redeem Israel. Not only this, but it is now the third day since these things [Jesus' crucifixion] happened." This verse took place on Sunday when Jesus was resurrected, and it shows the way they counted the days back then. They started counting with Friday as the first day, Saturday as the second, and then Sunday as the third day. Based on what we already showed, this period of time ending "on the third day" could also be written as something that occurred "for three days," since those are the same thing. So, even though it seems strange to us because our counting is different, in that day and age and culture, the phrase "for three days" really just meant parts of three days.