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)
The Merriam-Webster Collegiate Dictionary, eleventh edition, defines 'tertian' as 'recurring at approximately 48-hour intervals -- used of malaria'.
It could cause confusion, though. Since Latin 'tertius' means 'third', you would think it should mean 'every third day', and some dictionaries simply define it that way. But that's because the Romans included the current day in naming intervals. One of the dictionaries cited at https://www.definitions.net/definition/Tertian clarifies it by defining the noun 'tertian' thus: 'a disease, especially an intermittent fever, which returns every third day, reckoning inclusively, or in which the intermission lasts one day'. My italics.