After a little bit of research, I have confirmed that it would be correct to write “every so often” when the meaning you want is “every once a while.” Alternatively, you can refer to an event or occurrence as happening rarely or occasionally.
Take this example:
Every so often, I like to pack up my laptop and work from the coffee shop. I don’t do it all the time, but it’s nice having a change of scenery every once in a while.
This is the kind of usage that most of us know.
On the flip side, “ever so often” is a largely UK English phrase meaning “frequently” or “again and again.” Alternatively, you could just say that an event happens often. Here’s an example:
Road construction slows my commute home from work ever so often. I just can’t seem to avoid it.
As you can see, the meaning of “ever so often” is almost the complete opposite of “every so often.” There is only one letter that separates them, but as you’ve noticed, one letter can make all the world of difference. This can be more challenging to remember than the other bad habits that I hear ever so often.