According to the Collins Dictionary's definition:
If something happens every other day or every second day, for example, it happens one day, then does not happen the next day, then happens the day after that, and so on. You can also say that something happens every third week, every fourth year, and so on.
It would mean that if I see my friend today, then tomorrow a one-day-break, then the day after tomorrow I see her and it continues the same way, I can say each one of the sentences below interchangeably:
1.a. I see my friend on alternate days.
1.b. I see my friend every other day.
1.c. I see my friend every second day.
Also, if I see my friend today, then tomorrow and the day after tomorrow a two-day-break, then after two days I see her again and it continues the same way, I can say each one of the sentences below in an interchangeable way without any difference in meaning:
2.a. I see my friend every two days.
Or
I don't know if second is used here or in the former meaning (I mean: on alternate days)?
2.b. I see my friend every second day.
If "every second" is used in the meaning of "on alternate", then here I must use "every third":
2.c. I see my friend every third day.
And the rest is history.
On the other hand, someone else in this thread says:
"Every two days" is the same as "every other day." If you wanted to describe a two-day break, it would be "every third day.
This is what is not clear either in the similar thread on the forum, or in the Collins's definition.
Please let me know about it.
Best Answer
I'm not entirely sure what the question is, but I see no problem with 1a,1b,1c.
2a, and 2b however:
A two day break, and then seeing the friend on the day after, is every third day, (one day in three).
Every second day means a 2 day cycle, which is every other day
So 2c would be correct. (and that other thread would be correct too)
Attempting to be clear:
Every day with a visit = 1 day cycle = every single day = every day
One day empty, one day with a visit = 2 day cycle = every second day = once every two days = every other day
Two days empty, one day with a visit = 3 day cycle = every third day = once every three days
Three days empty, one day with a visit = 4 day cycle = every fourth day = once every four days
(the once can usually be omitted)