In the Book of Genesis, we read of appearances of angels and even of God, made to men.  These were preparations for the coming of The Son of God, our Lord Jesus Christ.  St. Albert the Great wrote a sermon for the season of Advent in which he described the biblical signs that would precede and accompany the life of our Lord here amongst us:

"There were multiple signs related to the redemption of man.  Of these, it is written in the present gospel: 'There will be signs in the sun, the moon, and the stars, and confusion of the nations on the earth'  (Luke 21, 25).  By 'the sun', the Son of God himself is understood, for he himself is 'the sun of justice', as it is written in Malachy 4, 2.  

1.  A great sign appeared in the sun in that it was darkened, as was foretold: 'The sun will be turned into darkness'  (Joel 2, 31).  Was not this 'sun' turned into "darkness", with respect to the Jews, when he was blindfolded, and, after being blindfolded, he was beaten and punched?  And when they said to him, 'Prophesy!  Who was it who hit you?' (Matthew 26, 68).  As though he did not know those who were hitting him.  Whatever splendor there was in the beauty of the face of this 'sun', it was obscured by the spittle of the Jews, who spat at him.  Matthew and Mark testify to this.  Thus also, in Isaiah 53, 2: 'There was no beauty in him, nor comeliness.'

2.  The Virgin Mary is signified by 'the moon'.  A certain saint says in her praise: 'It may be said that you are as beautiful as the moon, and there is merit in the comparison, for, of all the stars, the sun alone is brighter, and by its lustrous, silvery beauty, it outshines the rest of the stars in heaven.  By your glorious, virginal purity, you blaze forth among the myriads of angels serving God in the heights, as the clearest image of the true Sun.'

A wonderful sign was made in this 'moon', that is, the Virgin Mary: namely, that it was turned to blood.  This was foretold in Joel 2, 31:  'The moon will be turned to blood.'  The Virgin Mary was 'turned to blood' when she knew that her own Son had been betrayed, seized, beaten, spat upon, and demanded as a victim for the cross.  The whole 'moon' was made as blood when, assisting at the Cross, she saw the head of her Son lacerated by thorns, and his back, and every part of his Body, stained with blood; and when she saw his hands and feet pierced by nails, and, finally, his side cruelly opened with a lance.  Who is able to sufficiently describe how great we're the sorrows borne by the Virgin Mary?

3.  Signs occurred in the stars, as well.  The Apostles are understood by 'the stars'.  Just as the stars are fixed to the firmament, so they ever stayed fixed to Christ.  Truly, this sign appeared in the stars: namely, that they fell from the firmament.  In Matthew 26, 56, it is written that, 'all the disciples left him, and fled.'  Even Peter, who was more constant than the others, and followed him from a distance, was frightened by the voice of one serving maid, denied the One whom he followed with an oath.  As The Lord said, in Mark 12, 25: 'The stars of the sky fell.'

4.  There was also confusion in the lands.  By 'lands' [terrae] are understood those good women who followed the Lord, and wept and lamented over him, as it is written in Luke 23, 27.  For just as the earth [terra] nourishes men with the fruits which it produces, so did they feed Jesus as he went and preached throughout the land.  As it is written, in Luke 8, 23: 'Mary Magdalene, Joanna, the wife of Chusa, the steward of Herod, Susanna, and many other women who ministered to him out of their resources.'  When these women saw Jesus led to the punishment of death, they suffered the great confusion of sorrow in their hearts.  They suffered this, that is, on account of the cruelty of the Gentiles, who treated their Lord barbarously and wickedly.  

5.  There were also certain men among whom The Lord worked miracles, who seemed to wither out of fear.  They were afraid that the Jews might accuse them, inasmuch as Jesus had cured them in violation of the sabbath.  Their confusion was as great as the sound of the sea and its waves!

6.  Finally, 'the sea' signifies the elders of the .jews, namely, the priests and the chief priests.  The scribes and Pharisees are signified by 'the waves'.  All these had burst out in such rage and anger that they would have gladly erased the memory of Jesus from the earth.  So then they who had loved him before, seemed to wither out of their fear and anxiety of suffering the punishment of those men who had overcome 'the world', that is, Christ.  Christ is called 'the whole world' because, in him, we 'live and move and have our being,' as The Apostle said, in Acts 27, 28. Also, 'the powers, which are in heaven, will be moved.'  For, indeed, the sun, the moon, and the other stars withdrew their brilliance and became as darkness over the whole earth, from the sixth hour until the ninth, as is testified in Matthew 26, 45 and Luke 22, 44.  Behold, how many and how great were the signs that occurred at the time of human redemption!"