We must keep watch by night over the flock of our thoughts and actions. Then whether the Lord comes in the first watch or in the second or in the third, he will find us prepared.
The first watch is uprightness of action–trying to bring your life into line with the Rule you have vowed, not passing beyond the bounds that your forebears set to all the exercises of this way of life, not turning aside to the right or left.
The second watch is purity of intention, so that an artless eye will make the whole body light; whatever you do, you should do for God’s sake, and graces may return to the place from which they come to flow out again.
The third watch is the safeguarding of unity, so that situated as you are in community, you put what others want before what you want–so that you will stay among your brothers not only without quarrels but with grace, supporting them all, praying for them all. Then may be said of you, too: here is someone who loves his brothers and the people of Israel: here is someone who prays hard for the people and for the holy city of Jerusalem.
On this day, then, the coming of the Only-begotten enkindles in us true knowledge–the knowledge, I say, that teaches us that the Lord will come; and this knowledge shall be an everlasting and stable foundation for our way of life.
From Bernard of Clairvaux’s Third Sermon On the Eve of the Lord’s Nativity, Sermons for Advent and the Christmas Season, CF 51, pp 64-65