Complex sentence: Monthly and annual festivals were not usually allowed to fall on the same days so every festival month had an opening phase with exactly recurrent practices and celebrations while in the body of each month was a unique schedule of festival days.
Split into simpler sentences: Monthly and annual festivals were not usually allowed to fall on the same days. This means that every festival month had an opening phase with exactly recurrent practices and celebrations, while in the body of each month there was a unique schedule of festival days.