A routine appointment may be the event around which another person’s whole week is arranged.

The appointment you consider routine may be the fixed point of someone else’s week. The delivery, the shift, the standing visit, the short call: for you it is one item among many, and for them it is what the other days are arranged around. That asymmetry is invisible until you cancel and see the size of the reaction.

Before treating a small commitment as flexible, consider that its weight may be distributed unevenly. Give more notice than the thing seems to deserve. And notice which appointments in your own week are load-bearing, since it is worth knowing what is holding your structure up before something removes it.