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)