Learn English – A word for “every two days”

sesquiennial

ocurring every year and a half.

Etymology: semi ("half") + que ("and")

I cannot find a dictionary definition of this other than Wiktionary, and 0 hits on Google NGram.

But there are some examples of usage here

FAME now has responsibility for the sesquiennial (every 18 months) Music Festivals which attract players from Europe, American continent

Avoid biweekly altogether. Use fortnightly for "once every two weeks", and twice a week for, well, "twice a week".

Not everything has to be a single word, so don't be afraid to use more than one word when you want to use clear, understandable, unambiguous language.

If they have problems with fortnightly, use every other week, or let them into a little secret: fortnight comes from "fourteen nights", or two weeks.

Edit: From the comments it's clear many Americans won't understand or are uncomfortable with fortnight so, to be safe, use the aforementioned every other week.