The description of the Book of Days in the Southern Cost description in the Ruins of the Empire is extremely vague. To my knowledge that is the only reference to it in the boxed set. Is it covered in more detail in another publication?
Either way, have you used in in your campaigns? If so what exactly is it?
I have not yet used it in my campaign but from what I read it is a powerful artifact that contains all the info about everything. I was thinking about using it in my campaign, those who read it improve their arcana skill and increase one point in intelligence. I was also thinking that it would be useful some how in reclaiming the Iron Throne but I have thought it all out yet.
I think it is also briefly mentioned in the BoP under the Haelyn descripton, but I may be mixing that up with the Church in Aerele, which is where it is kept.
You would be confusing it with the Book of Law which is the holy concordance of the Haelynites.
And as for the Book of Days being stored in the Church in Aerele…nope again, it was stolen just at the end of the published timeline in the RoE.
Who is confusing it with the book of Law?
Why, Magian, of course. Unless I scanned too quickly, the Book of Law is mentioned during the Haelyn discourse of the BoP, and NOT the Book of days.
from what i read is the book of days the exact calander of the anuireans based on astrology.
I make The Book of Days and The Book of Law two parts of the same THREE part Holy Book of Three (aka Holy Book of Haelyn (see below), which is the equivalent of the Bible in Cerilia; B of Days (old testament) B of Laws (new testament) with an “unmentioned” Book of Light.
The Book of Days is the chronicle of the Battle of Deismaar and the years surrounding the sundering of the land IMO. The Book of Law (again, IMO) came about when Haelyn made himself known and preached the “new rules” pertinent to the priesthood and ALL faithful worshippers of the new gods (as opposed to His - Haelyn’s - dictates). In the later half of the Book of Laws, Haelyn then sets out His own Code.
I generally muddy the waters by adding in Avani’s equivalent “The Book of Light”, with each religion saying theirs came out first and theirs is the “True Word”. Adds a nice touch of conflict ala our own Earth religious conflict surrounding the Bible. Its interesting to note that both Haelyn and Avani claim the sun as part of their “symbology” and I use that IMC for good effect.
I claim Haelyn takes responsibility for the Holy Book of Three being as he seems to be the ‘leader’ of the good gods. Not to mention, I gave the Holy Book three parts not in small part as a reference to the alignment system of AD&D. 
I have used the Book of Days as a backdrop for an entire mini-campaign. There is only some vagure hints in the RoE book, so I made up something.
Like arjan, I have the The Book of Days as a work of astronomical observations - stretching back to ancient times. Having access to these observations allows the right kind of spellcaster the ability to predict the future rather acurately. Made a nice backdrop for many adventures as the players tried to located the Book (which is really multiple volumes) before some unknown factor used it to win the Iron Throne (very original).
Has anyone else made up what the book is about and can they tell us of how it fits in to their campaign?
In a campaign I’m in now, everyone is after it. Noone is sure what it does. One group wants it because it can tell the future. Another one wants it because they believe it to be an all powerful speelbook. There are like 10 different groups after it, for ten different reasons. My character wants it to find a family artifact, and as a bargaining chip with the other groups. I think they left it vague for just this reason. To let DM’s drive players crazy with the unknown.
We’ll probably find it and it will turn out to be a simple calender figured out to 2,000 years. Not magical at all. lol.