Sayings of the Holy Fathers:

"Full of meaning is also the addition `of this day,' when He says: `Give us this day our daily bread.' These words contain yet another teaching. For you should learn through what you say that the human life is but the life of a day. Only the present each one of us can call his own; the hope of the future is uncertain, for we know not `what the day to come may bring forth.' Why then do we make ourselves miserable worrying about the future? He says, `Sufficient for the day is the evil thereof,' evil here means the enduring of evil. Why are we disturbed about the morrow? By the very fact that He gives you the commandment for today, He forbids you to be solicitous for the morrow. He says to you as it were: He Who gives you the day will give you also the things necessary for the day."

St. Gregory of Nyssa