Members of this species are relatively pale, and among the largest in subgenus Liocosmius. The head, pronotum, and abdomen are relatively pale brown or reddish-brown (in contrast to the dark brown or black colors of all other Liocosmius except B. "Cooper's"). The elytral margins are pale. The hind elytral spots are distinctly comma-shaped. The antennae are quite pale, either yellowish brown or pale reddish. The male genitalia have a very large, sinuate flagellum, and in this are distinct from all other Liocosmius, except B. "Cooper's".
There are no obvious differences between B. festivum as a whole and B. "Cooper's" in external appearance except for the slightly more obtuse hand angles of the pronotum in B. festivum. However, the individual subspecies of B. festivum can be told from B. "Cooper's" more easily: specimens pale prothoraces, large sinuate flagella, and with more than two discal setae on each elytron or with elytral microsculpture are B. festivum. If it were not for the notable differences in DNA sequences, these might be considered to belong to the same species.
Body length 3.5-4.5 mm, with most specimens 3.8-4.2 mm.