What does saltus lūnae mean in Latin?

saltus lūnae, a noun, means “the additional day added to a Metonic cycle in computus to bring the solar and lunar calendars into alignment”.

What is the genitive of saltus lūnae?

The genitive singular is saltūs lūnae. saltus lūnae is fourth declension.

nounfourth declension

saltus lūnae m (genitive saltūs lūnae)

the additional day added to a Metonic cycle in computus to bring the solar and lunar calendars into alignment

From saltus (“leap”) + lūna (“moon”).