Learn English – Unambiguous adjective for an event that happens once every two months

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)}

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