Serenity - a distinctive feature of St. Ursula that she demanded of the Ursuline Sisters of the Agonizing Heart of Jesus. The feature grows out of the experience of God’s love as well as of awareness that Christ raised us from death to life and promised that He is with us till the end of time. Serenity experiences a specific "soul’s happiness", gratitude and joy, deep optimism, which stems from faith and hope.

Only the person in harmony with the will of God, seeking happiness in the will of God, in God's bright spheres, will be able in spite of everything to preserve within herself that holy serenity, that bright, sunny happiness of God. 
St. Ursula, Testament, Twelfth Request

The sunny person is by herself an apostle. She unknowingly evangelizes and leads people to God. She tells them without words, but by her own bright smile, that it is good, very good to serve God and that to serve God is to find happiness and peace such as the world cannot give. St. Ursula, Testament, Twelfth Request

Ursuline serenity results in willingness to fulfill God's will – in Christ’s likeness during His agony in the Garden of Gethsemane as well as to resemble Mary during the Annunciation, certain that I am in hands of good God. «As God wills it». These words uttered with love are balm for the distressed soul. Utter them in times of worry and anxiety, utter them also when God sends a cross upon you. Oh, then do not only reconcile yourselves to the will of God, but also submit to it quietly, without complaint, with a smile on your lips and «Deo gratias» in your hearts. St. Ursula, Testament, Eleventh Request

So be sunny, bright and serene, my children. You will thereby delight the Heart of God and (…) you will be willingly and cheerfully fulfilling the will of God, which is, in the words of St. Paul, your sanctification. So always cheerfully, serenely, sunnily, and with confidence up to God in heaven!  St. Ursula, Testament, Twelfth Request