Et valde mane is found both before and after the reform of Pius XII as the antiphon to the Benedictus at Lauds of Easter Sunday. In the post Pius XII Easter Vigil, a brief form of Lauds is sung at the conclusion, including this antiphon (Liber Usualis [1962], p. 776KK). The pre Pius XII Easter Vigil took place earlier in the day, and so it was concluded with Vespers, rather than Lauds, and had a different antiphon; still, Et valde mane is found in Lauds, which comes after Matins of Easter Sunday, as the same antiphon to the Benedictus (Liber Usualis [1950], p. 777.
In the OF Liturgy of the Hours, the antiphon Et valde mane is sung at the Benedictus at Lauds of Easter Sunday of Year B. See the free booklet on my website, p. 9. Source for the antiphon is the 1934 Antiphonale Monasticum, p. 456.
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