Planning your day is essential if you want to be able to have deep focus and get things done. When you fail to schedule the time to work on projects, it's very easy to be swayed by the issues of the day. You know, those things that are urgent and important for others, but not necessarily for you. Instead, you want to focus on those not urgent and important things that ultimately help you reach your life goals.
I plan my time in two steps, going from coarse- to fine-grained. During the coarse-grained phase at the start of each week (or better yet, at the end of the week before), I look at my calendar for the upcoming week. For the already scheduled meetings, I schedule the time needed to prepare them. The fine-grained phase happens at the start of each day. I will review the meetings and other blocks of time I already scheduled and update them if necessary.
My most productive hours of the day are in the morning, so I schedule all deep- and creative work during that timeframe as much as possible. The afternoon is reserved for the small, shallow, and ad-hoc tasks like merge requests and email.