The period during which He was to lie in the grave is here expressed in round numbers, according to the Jewish way of speaking, which was to regard any part of a day, however small, included within a period of days, as a full day. (See 1Sa 30:12, 13; Es 4:16; 5:1; Mt 27:63, 64, &c.). (JFB)
“Three days and three nights” is from Jnh 1:17. In Jewish reckoning, a period of light and darkness makes a “day” (onah), and any part of a “day” is considered as the whole. Since Jesus arose on the third day (Mt 16:21; 27:63–64), which was the first day of the week (28:1), the language of three days and nights does not mean 72 hours (see Est 4:16; 5:1) but “part of Friday, all of Saturday, and part of Sunday.” (Apologetics Study Bible)