Bava is the first karana. A karana is half a tithi, the time the Moon takes to gain six degrees on the Sun, so there are two karanas to each lunar day and sixty in a month. The scheme uses eleven named karanas: seven that recur in rotation, the chara or movable karanas, and four that appear only once each, the sthira or fixed karanas clustered around the new moon. Bava is the first of the seven movable karanas, and through the month it recurs eight times. As one of the moving karanas it is regarded in the panchanga as generally benefic, suitable for auspicious and stable undertakings. The karana is one of the five limbs (anga) of the panchanga, the daily Hindu almanac, alongside the tithi, nakshatra, yoga, and weekday. These are the calendar tradition's conventions, resting on the six-degree steps of the Moon away from the Sun.
Varahamihira's sixth century Brihat Samhita is among the earliest surviving texts to treat muhurta, the selection of auspicious moments, in systematic detail, and the karanas take their place there alongside the tithi and nakshatra as one of the factors an electional astrologer weighs; the later Muhurta Chintamani of Daivajna Ramachandra, composed around the turn of the seventeenth century, is the more commonly cited classical handbook in contemporary practice for the detailed rules governing which karana suits which activity. Both belong to a long continuous tradition of muhurta literature in which the karana, as the finest-grained of the five daily limbs, is typically the last factor checked once the coarser tithi and weekday have already been judged favourable.
Sources Muhurta ChintamaniRama Daivajna with Brihat SamhitaVarahamihira