How far in advance should I actually plan my calendar?

There's no single right answer, because different kinds of commitments deserve different planning horizons. Planning everything the same distance out, whether that's a day or a year, usually wastes effort in one direction or leaves you exposed in the other.

Fixed, unchangeable commitments: plan as far out as you know them

Things that are already decided and unlikely to move, a flight, a wedding, a recurring weekly meeting, should go on the calendar the moment you know about them, regardless of how far out that is. There's no cost to having these visible months in advance, and real cost to forgetting them because they weren't written down early.

Recurring commitments: set once, they don't need re-planning at all

Anything on a fixed weekly or monthly cadence shouldn't be "planned" repeatedly. Set it up once as a recurring event and it's permanently on your calendar going forward, with zero additional planning effort required for each future occurrence.

Flexible tasks and plans: a week or two out is usually the sweet spot

For things that aren't fixed, when to run errands, when to catch up with a friend, planning much further than a week or two out tends to be wasted effort, because life shifts and those plans often get rearranged anyway. A rough week-ahead plan, revisited and adjusted as the week actually unfolds, tends to be more accurate than a detailed month-ahead plan that gets rewritten repeatedly.

Big, irregular goals: plan the milestones, not the daily detail

For longer-term things, a big project, a personal goal, it's worth marking key milestones or checkpoints on the calendar months ahead, but not trying to plan the daily specifics that far out. The daily detail should be filled in closer to the time, once you actually know what the days around it look like.

A simple rule of thumb

Plan fixed things as far ahead as you know them. Plan flexible things about a week or two out. Let recurring things plan themselves. And for anything further out than that, a placeholder or milestone is more useful than a detailed plan you'll likely have to redo anyway. Over-planning the distant future usually just means re-planning it multiple times before it arrives, which is more effort than planning it once, closer to when it's actually happening.