Exactly as the title says, been in to the sewers both during the day and night but have found nothing!

At the top right-hand corner of the map in the sewers, beyond the room with the metal grille doors. There is a desk and a trapped chest in that room. The diary is on the desk and will either enter your inventory or drop on the ground when you interact with the desk.

And to get to that room you must pull total of 3 candle ( or something like that) to make the doors open