Easter Letter

Hello there!!!...I am translating an english book into a spanish and I've come across some words I just can't find the appropiate spanish language ones. For instance, What's "Easter Letter" in Spanish?...Here you have the context:

Our New Testament Canon was first listed by
the very same Athanasius, after he became a
bishop, in his Easter Letter of 367, and it was
universally confirmed at the Council of​
Ephesus in 431.

Thanks in advance
    Well, I had Carta de Pascua but I was looking for something more than just the literal meaning!!...I think I might goggle that! Thanks:)