"God helped her in the early morning light; the Most High sanctified His dwelling." El Greco's masterpiece, with its refulgence of both the sun and the moon, conveys the feeling of this antiphon from 2nd nocturn of the feast. Also this response to the 4th lesson: "I made an unfailing light arise in the heavens." And yet more: ""She is the splendour of the eternal light and the mirror unspotted. For she is more beautiful than the sun, she is purer than light itself." (response to 5th lesson) The painting furthermore contains imagery from a homily by Saint Germain: The ship at lower right alludes to the "port of salvation;" the "fragrant lily and unfading rose;" the "palace wherein is the spiritual bridal chamber;" and the "fountain of flowing waters."
There is a close connection between the Immaculate Conception and the Assumption. Both feasts celebrate the fact that she offered no resistance to divine grace. And El Greco's painting celebrates that connection. Consider the following verses from the Liturgy of the Hours for the Feast of the Assumption: "Mary has been taken up into heaven! The angels are rejoicing;" "The holy Mother of God has been exalted above choirs of angels."