Gravissimum educationis is the Second Vatican Council's Declaration on Christian Education. It was promulgated on 28 October 1965 by Pope Paul VI, following approval by the assembled bishops by a vote of 2,290 to 35.
The document is generally referred to, not by its English-language title, "Declaration on Christian Education", but by the opening words of its original Latin text, which mean "extremely important education".
The document quotes at length from Divini illius Magistri (That divine teacher, 1929) by Pope Pius XI.