This is a good example of a “tradition of men” which has no Biblical support whatsoever.  I have read that this was one of the traditions of the Eastern Orthodox churches as well as some Roman Catholic churches. They believe that “the 40th day is a key time of the soul’s journey, often commemorating the period after Jesus’s resurrection before His ascension” (see Acts 1:3). But nothing is said about “praying for the dead”; rather we read that “He (Jesus) presented Himself alive after His suffering by many infallible proofs, being seen by them forty days and speaking of the things pertaining to the kingdom of God.” Jesus spoke to them about the kingdom of God, not about praying for those who have already gone out into eternity through the portal of death.

Others who may practice this “tradition” point to the fact that “the number 40 is a recurring theme in the Bible for significant periods of transition.” It is true that there are multiple instances of this in Scripture, but none of them have anything to do with “praying for the dead” during that transition. Here are examples of 40 days in Scripture:

*Genesis 7:12: “And the rain was on the earth forty days and forty nights” (The flood in Noah’s day).

*Exodus 24:18: “So Moses went into the midst of the cloud and went up into the mountain. And Moses was on the mountain forty days and forty nights” (Moses received the Ten Commandments.)

*Deuteronomy 9:18 & 25-26: “And I fell down before the LORD, as at the first, forty days and forty nights…. I prostrated myself before the LORD;  and forty nights I kept prostrating myself, because the LORD had said He would destroy you. Therefore I prayed to the LORD, and said: ‘O Lord God, do not destroy Your people and Your inheritance whom You have redeemed through Your greatness, whom You have brought out of Egypt with a mighty hand.” (Moses praying “for the LIVING, not for the DEAD.)

*1 Kings 19:8: “So he arose, and ate and drank; and he went in the strength of that food forty days and forty nights as far as Horeb, the mountain of God” (Elijah the prophet who had been depressed and then God fed him which enabled him to travel 200 miles to Mt. Horeb in 40 days.)

*Matthew 4:1-2: “Then Jesus was led up by the Spirit into the wilderness to be tempted by the devil. And when He had fasted forty days and forty nights, afterward He was hungry.”

Again, there is not a word about “praying for the dead” in these examples. The number 40 seems to indicate a “time of TESTING.”  I assume some are “praying for the dead to be SAVED” which suggests they knew they had died UNSAVED and they are hoping their prayers will move the heart of God to spare them from eternal punishment and bring them to eternal life in heaven. If this is what they have in mind, they need to read Luke 16:19-30 where Jesus told the true story of two men (a rich man and a beggar named Lazarus). They both died, were buried, and their spirit and soul went out into eternity. Lazarus was “carried by angels to Abraham’s bosom” (verse 22) and the rich man was “in torments in Hades” (verse 23). We do NOT read of anyone “praying for the rich man to be saved from his torment in Hades.” He was told “there is great gulf fixed” (verse 26), which means their eternal destiny was FIXED for all eternity! After hearing that we actually read of him praying for Abraham to send Lazarus back to earth to testify to his brothers, “lest they also come to this place of torment” (verse 28). Abraham’s response was, “They have Moses and the prophets; let them hear them…If they do no hear Moses and the prophets, neither will they be persuaded though one rise from the dead” (verses 29 & 31). The lesson is clear: once a person dies, they either go to heaven or a place of torment. If one dies unsaved, no one can “pray them out of Hades”; their eternal doom is sealed! They had a chance to hear God’s word and be saved but they turned a deaf ear to God’s message of salvation and are now lost forever.  (DO)  (706.3)