The body thrives on regularity, from your eating, your sleeping and especially your training. I have always had a set practice time; from the earliest days of gymnastics to my 5 am running jaunts to my bodybuilding and powerlifting workouts.
Knowing exactly when you are going to workout makes it so much easier to plan, visualize and mentally rehearse what you are going to do the next workout. I can't beleive how many show up at the gym not really knowing what the days session is about. Now this make work alright for 'exercisers' but for those serious about making progress it is not okay at all.
Back when I was still training for competition I knew weeks in advance what my workouts would be as I had to to be able to peak for my competition. If my goal was to squat, bench or deadlift a certain amount then all the workouts leading up to that had to be sequential and planned as well.
If, for instance a workout happened to fall on Thankgiving, or Christmas or my birthday, oh well, had to "get er dun".It was only though this total committment to my goals did I have a chance of making them happen. Having equally committed( nutso) training partners really helps here and they are harder to find than a woman who will put up with you.
But if you can't find those partners you have to get er dun yourself, regardless. It's not really a plan if you don't follow it and you'll never know if your training theories are working if you keep changing things.
For those that squeeze their workouts in when everything else is done really haven't prioritized their training correctly ,imo. If you plan to train at midnight or 5 am so as not to interfere with your family time thats fine as long as there is a plan and it's stuck to.
We make progress in the gym one day at a time, one workout at a time and one rep at a time.Committing fully to each aspect of your training makes you stronger each time you do it. Or not.