The phrase "in the day" means during the daytime or during a particular time period. It can also refer to something that happens commonly, usually, or typically within a given time frame.
That was back in my days of eating white bread topped with margarine topped with sugar. (ancestral-nutrition.com)
To make it work, she simply moved that component to a time later in the day when she knew that she would have everyone back in the classroom. (edutopia.org)
Because if they meant something important back in the days of the early church, they can mean something today. (redeeminggod.com)