According to Enya.sk, Enya said in a 1993 interview for ITV:

Well there is a strong theme of ‘book of days’, because I have a book of days, a diary. And I’ve always kept one. And it was Roma)’s suggestion again. Ah, she likes to be able to associate with me, because I’m the one who’s going to sing. There is some Gaelic in it.. in the instrumental version.. and it talks about the beginning of the day and the excitement, the expectations, one has. And it’s the eagerness of what is to become of this day. And when Ron Howard approached us, for the film Far and Away, it was similar in a way because there were these two people, and we watch their ‘book of days’, their diary, of how they met up and how they went across to America. And so it was quite easy to interpret the lyrics with the song, because it was still associated with the theme that was originally there.

Lyricist Roma Ryan has written in the album’s music book:

I am collecting diaries more and more and I continue to be intrigued by thepossibilities of each blank page – what will it hold? I experience a mixture of sadness and exuberance when I think of lives laid out on a page or in a book. Personal histories of great loves, of journeys exotic and wonderful, of loss, fear and tragedy, of dreams and simple wishes.

On Enya’s YouTube channel, in the comments of the video for the Shepherd Moons 30th birthday, she wrote:

The original version of Book of days is the Gaelic one, which you will also hear in this Anniversary watching party. Ron Howard, who had chosen the track for his film Far and Away which as you can see by the footage, stars Tom Cruise and Nicole Kidman, had asked us to adapt the piece to his particular vision of the scene. We found him to be a very sincere man who knew exactly what he wanted for his film, and, in this way, this version of the song was born.