See this online text:
Sermons on the Feast of the Nativity, by Saint Leo the Great / Pope Leo I, comprising Sermons XXI-XXIV and XXVI-XXVIII, on pages 128 – 144, of:
The Letters and Sermons of St. Leo the Great, Bishop of Rome, translated, with notes, introduction, and indices, by Charles Lett Feltoe, in A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, translated into English with Prolegomena and Explanatory Notes, under the editorial supervision of Philip Schaff and Henry Wace (New York: The Christian Literature Company; Oxford, London: Parker and Company, 1895), Volume XII, Leo the Great. Gregory the Great. The volume is available in pdf, mobi, epub and other formats at Internet Archive.
The text of the individual sermons, as revised by Kevin Knight, indexed as the Sermons of St. Leo the Great, is available at New Advent’s Fathers of the Church collection: Sermon 21; Sermon 22; Sermon 23; Sermon 24; Sermon 26; Sermon 27; and Sermon 28.
The text of the individual sermons is also available at Catholic Culture: Sermon XXI: on the Feast of the Nativity, I; Sermon XXII: on the Feast of the Nativity, II; Sermon XXIII: on the Feast of the Nativity, III; Sermon XXIV: on the Feast of the Nativity, IV; Sermon XXVI: on the Feast of the Nativity, VI; Sermon XXVII: on the Feast of the Nativity, VII; and Sermon XXVIII: on the Festival of the Nativity, VIII.
On the Feast of the Nativity of our divine Lord Jesus Christ in the one true and Catholic Faith. For other texts and ebooks, you may access the List of Free eBooks (Arranged by Title), the List of Free eBooks (Grouped by Subject), the List of Worth-It Catholic Books & eBooks, and the main page of the Catholic eBooks Project.