In like manner also we can understand this statement: “This month shall be to you the beginning of months.” This statement is to be interpreted in reference to time. There is reference to the pasch of the Lord, which is celebrated at the beginning of spring. Therefore he created heaven and earth at the time when the months began, from which time it is fitting that the world took its rise. Then there was the mild temperature of spring, a season suitable for all things.
After having inflicted the Egyptians with many plagues, he led them out of Egypt in the month of flowers, when the most pleasant spring appears and the sadness of winter passes away.
This, I say, is the first month of the year. This brings joy to every creature. It clothes the naked trees. It opens the earth. This produces joy in all animals. It brings mirth to all. This is for Christians Xanthicus, the first month, the time of the resurrection in which their bodies will be glorified by means of the light which even now is in them hidden. This is the power of the Spirit who will then be their clothing, food, drink, exultation, gladness, peace, adornment and eternal life.
Consequently our elders decided that one full month must be observed for the birthday of the world and that Easter should be observed in whatever part of it both the day and the moon coincided. This is not without scriptural authority, for Moses said, “This month shall stand at the head of your calendar, the first month of the year.” With these words he consecrated a whole month for the day of the world’s birth. Thus our elders, who had found that March 22 was the birthday of the world, defined April 21 as a limit in determining the first month. So it will be permitted to celebrate Easter neither before March 22 nor after April 21. But when during this month both the moon and the day coincide, that is, the fourteenth day of the moon and Sunday, then Easter is to be celebrated. Now again, since the fourteenth day of the moon frequently does not fall on Sunday, they preferred to have the moon extended for seven days, provided they observed Sunday in the joy of the resurrection. So when the day falls thus, we always postpone Easter as far as the twenty-first day of the moon for the sake of Sunday, so that Easter is celebrated neither before March 22 nor after April 21. In this way it is found that the month and the day and the moon are retained in the observance of Easter.
[AD 397] Ambrose of Milan on Exodus 12:2