The door of the sky (Early Modern English: the gate of Heaven) it’s either a expression which is used literally to refer to the rain pouring from the clouds or metaphorically for prayers and good doings.
The plural forms the doors of the skies (EME: the gates of the heavens) are also used.
In canonical Islam, Christianity, and Judaism, there is no door or gate in the sky, the physical cosmos, or the overworld, but there is one gate in Judaeo-Christian Heaven/Paradise and eight in the Islamic one.