Skeptics question why Jesus was in the tomb for only two and a half days at the most, when he said he would be there three days and three nights (Matthew 12:40).

This is actually an instance in which we need to understand Jewish idiom, which understood "a day and a night" to include even the smallest part of a day and night. A Jewish source from after the time of the New Testament puts it this way: "A day and a night are an Onah ['a portion of time'] and the portion of an Onah is as the whole of it" [J.Talmud, Shabbath 9.3 and b.Talmud, Pesahim 4a]. Other examples of this kind of usage can be found throughout the Bible (Genesis 42:16, 1 Kings 20:29, Esther 4:16, Matthew 27:63). Jesus was in the tomb for only a small part of Friday and Sunday, but that counts according to Jewish idiom for the entire "day and night" for each of those days.

The fuller cite from the Talmud reads:
> Weigh well that which is disputed in the tract Schabbath, concerning the uncleanness of a woman for three days; where many things are discussed by the Gemarists concerning the computation of this space of three days. Among other things, these words occur; "R. Ismael saith, Sometimes it contains four Onoth, sometimes five, sometimes six. But how much is the space of an Onah? R. Jochanon saith either a day or a night." And so also the Jerusalem Talmud; "R. Akiba fixed a day for an Onah, and a night for an Onah: but the tradition is, that R. Eliezar Ben Azariah said, A day and a night make an Onah, and a part of an Onah is as a whole." And a little after, R. Ismael computeth a part of the Onah for the whole. (p. 210, vol. 2, Commentary on the New Testament from the Talmud and Hebraica)

As an aside, a counter-argument sometimes set up against this is that because the reason for the discussion in the Talmud is a menstrual cycle, then this designation applies only for menstrual cycles. That is not the case. While the menstrual cycle was the issue causing the discussion, the resolution is completely independent of the problem and is not uniquely associated with it.

In addition, the online Jewish Encyclopedia says this about "day":
> In the Bible, the season of light (Gen. i. 5), lasting "from dawn [lit. 'the rising of the morning'] to the coming forth of the stars" (Neh. iv. 15, 17). The term "day" is used also to denote a period of twenty-four hours (Ex. xxi. 21). In Jewish communal life part of a day is at times reckoned as one day; e.g., the day of the funeral, even when the latter takes place late in the afternoon, is counted as the first of the seven days of mourning; a short time in the morning of the seventh day is counted as the seventh day; circumcision takes place on the eighth day, even though of the first day only a few minutes remained after the birth of the child, these being counted as one day.

Pinchas Lapide's The Resurrection of Jesus (91-2) takes the reference to the "third day" as alluding to various passages in the OT where after a "third day" something happens in the "history of salvation," (Genesis 22:4, 42:18; Exodus 19:16; Jonah 1:17; Esther 5:1; Hosea 6:2) and thus contains for ears which are educated biblically a clear reference to God's mercy and grace which is revealed after two days of affliction and death by way of redemption. In other words, it is a recognized literary and cultural idiom, and thus must be read within that historical context.