As far as I know,

every other day = every second day

and I think (but I'm not sure) that they are both common. Is every two days also correct and common to use instead of them? I am also wondering whether

every three days = every third day

and if they are used commonly?

  • You should try to pin this down using a Google Ngram first, then add the details to your question if it is still unclear. – user3169 Sep 9 '17 at 20:56
  • Can Google Ngram be trusted? I am asking this because I used to add the number of hints (exact matches) given by Google search engine in my questions but one time people commented (very seriously) that those results are obtained through some weird algorithm and can not be trusted! – r.beigi Sep 10 '17 at 1:36
  • No, but it is a start. Then you can consider detailed contexts. Otherwise, I think you will only get opinions. – user3169 Sep 10 '17 at 1:55

My instinct is that "every other day" is vastly more common than "every second day" - and NGrams confirms this. Both are correct, though.

(It is interesting to note that the use of "other" to mean "second" goes right back to Old English, whereas the word "second" is much more recent, not attested until 1297. Still, there are many contexts in modern English in which only "second" is permissible. You can come first or second in a race, for example, but you can't come "other".)

"Every three days" and "every third day" are both acceptable too, and here the difference in frequency is much smaller, but recently "every three days" has had the edge, according to NGrams.

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