9 That this number, then, is a sign of that laborious period in which, under the discipline of Christ the King, we have to fight against the devil, is also indicated by the fact that both the law and the prophets solemnized a fast of forty days—that is to say, a humbling of the soul—in the person of Moses and Elias, who fasted each for a space of forty days. And what else does the Gospel narrative shadow forth under the fast of the Lord Himself, during which forty days He was also tempted of the devil, than that condition of temptation which appertains to us through all the space of this age, and which He bore in the flesh which He condescended to take to Himself from our mortality?
After the resurrection also, it was His will to remain with His disciples on the earth not longer than forty days, continuing to mingle for that space of time with this life of theirs in the way of human intercourse, and partaking along with them of the food needful for mortal men, although He Himself was to die no more; and all this was done with the view of signifying to them through these forty days, that although His presence should be hidden from their eyes, He would yet fulfil what He promised when He said, “Lo, I am with you, even to the end of the world.” And in explanation of the circumstance that this particular number should denote this temporal and earthly life, what suggests itself most immediately in the meantime, although there may be another and subtler method of accounting for it, is the consideration that the seasons of the years also revolve in four successive alternations, and that the world itself has its bounds determined by four divisions, which Scripture sometimes designates by the names of the winds—East and West, Aquilo [or North] and Meridian [or South]. But the number forty is equivalent to four times ten. Furthermore, the number ten itself is made up by adding the several numbers in succession from one up to four together.
Source: The Harmony of the Gospels (New Advent)