I think trying to plan an entire month on a fine-grained day-by-day level of detail would be both an enormous amount of work and largely useless due to unpredictable changes.
I'm lucky to have a pretty flexible job, so my monthly plan basically consists of:
- a few inflexible events on my calendar every week (mostly meetings and such, some of which require a few hours' or occasionally a few days' preparation)
- a list of tasks ordered by urgency, with an estimate of how long they should take (a few days, a week, 2-3 weeks, etc)
So every day I deal with the inflexible events as they show up, and spend most of my time going down the list of tasks in priority order (of course, the priorities may change or something else may get added). If I have multiple tasks with about the same priority, which happens often, I work on whatever I feel like that day, and try not to context-switch too often.
If a task goes significantly over its duration estimate, then I need to think if it's a good idea to keep going - usually I try to get to some sensible stopping-point that qualifies as having gotten something done on the issue, and postpone it in favor of hopefully quicker things (or not, if it's much more urgent than everything else), and wait for a chance to discuss it with my boss.