What is the role of "every" in idioms like "every so often"?

There are a couple of idioms whose meaning is from time to time or occasionally.

Every so often
(Every) once in a while
(Every) now and then/again

Every actually is a determiner (or, broadly speaking, an adjective) specifying a noun or noun phrase and means that everybody or everything of the referred group is addressed, as in every day, every man, or every student.

Obviously, every is also used when referring to a period or range of time, e.g., every thirty minutes or every week but, again, the time frame is given as a noun.

I was looking at the entry in Etymology Online, as well at Merriam-Webster and Oxford dictionaries to get further information.

Etymology Online doesn't address the matter at all. While the dictionaries at least define these phrases and list a couple of examples, they do not give any clue to answer my questions: what part of role is every playing in these expressions (from both a grammatical and logical point of view) and where did this usage come from.

Solution 1:

You've answered your own question: every is a determiner which is used to introduce the frequency of the action. The difference is that some actions are rare or infrequent or the period isn't strictly known.

• every day : once a day
• every 5 minutes: repeats with a period of 5 minutes
• every once in a while: repeats with an unspecified period (a while)
• every so often: repeats with an unspecified period
• every once in a blue moon: repeats on rare occasions when the moon is blue; the time period is the amount of time between blue moons. (This usage is never meant to be taken literally - it just means "rarely").

Just because "once in a while" is a phrase doesn't make it function like less of a noun than "day" or "5 minutes". In all of those cases, the word or phrase after "every" represents a time period.

Solution 2:

It is an expression that everyone knows and uses. It's just a convention now. It isn't necessarily correct or logical. In fact, it is redundant. You could remove Every from each example you listed and it would still make perfect sense: Often, Once in a while, Now and then/again.

It's the same things as people saying "general consensus". General is totally unnecessary because "general" is already inherent in the meaning of consensus. It's just the habit of people to use 5 words when 2 will do.

Wordiness is often mistaken for eloquence because "every so often" is smoother than just "often", but there is no difference in the meaning. And if you are trying to be succinct and concise then you might avoid those flourishes.

I don't know about it's origin though.