A word for "every two days"

Is there an adjective that means "every two days", i.e. is to a day as biennial is to a year?

In regular conversation, the phrase is simply every other day. Technically, however, one could use bidiurnal. It appears the word may have been coined by Ursula M. Cowgill in her 1965 paper, A bidiurnal cycle in the feeding habit of Perodicticus potto, from which I quote thus (emphases mine):

A definite bidiurnal cycle exists; the data corrected for natural loss are shown in Figure 1. The χ² for the 48-hr periodicity is highly significant (χ² = 188).
_{Cowgill, PNAS, 420 (1965)}

You could use "alternate".

I used to post on Instagram daily, but now I've switched to posting on alternate days. P.S Correct me if I'm wrong. 😅

The abbreviation QOD or QAD (from Latin mean Quaque Alternis Die") means 'every other day' or 'every two days'.

Modern style in medical situations recommends using using the spelled out English 'every other day' because the Latin abbreviations are often misread.