Civil law is one of those subjects where reading the book once just isn't enough - you need to build a mental map of the concepts and how they connect to each other. Start making summaries by topic right away (don't copy from the book, write in your own words) and then go back over your summaries a week later to check if you actually remember. If you've missed lectures, ask someone for their notes and read the textbook first on that topic - context helps you avoid forgetting. A month is plenty of time if you study systematically, not at the last minute.