Hi! Every two days would be correct. You can view "two days" as a singular block, not two separate days. Similarly you could look at it like a single five year block in "every five years" etc. So if I were to say, "every two weeks, I vacuum my apartment", you can try to visualize blocks of two-week periods, in between which I do the vacuuming. That way the rule of every being followed by singular nouns still holds, at least when picturing the meaning mentally.

Thank you!

Pasted as an answer instead :)

Hi Phew :) Yes, "every two days" is correct. You're correct that "every" is usually followed by singular nouns, but this is a special case where the noun is described by a plural adjective, and the two must match. You can't ever say "two day." It has to be "two days."