Comment by zdc1

"I go to the dentist every year" implies that you have a usual dentist or some sort of stable routine or plan vs "I go on a trip every year" which implies the journey is yet to be selected/defined from a multitude of options.

So I feel the rule isn't always taken literally, but the vs a is selected based on whether something is up for consideration/substitution rather than something that's well defined and specified.