The excavation days. The day you start, and you have the drawings, and you know how deep you are going into the ground; you might find something unexpected, and you have to change your plans. I love that challenge.
I work for highway maintenance, so we look at the engineering elements of planning drawings and carry them out. There are so many different elements of engineering within different projects, you must know and understand the drawings before you do anything else. Sometimes this could be the renovation of an entire street, which could include new ducting, new electrical cables etc.