Pradosh Kaal in Hinduism is the twilight period. The
twilight period on the 13th day during the waxing and waning phase
of moon (Shukla Paksha and Krishna Paksha) is the Pradosham period. Golden rule is
that Trayodashi tithi should be present during twilight period. If not Pradosh
kaal is observed a day earlier or a day later.
The popular belief is that Lord Shiva and Goddess Parvati
are at their happiest mood during the period and they will bless devotees
during the period. Desires are fulfilled when prayers are observed during the
Pradosh Kaal.
Roughly it can be stated that Pradosh Kaal is the period 1.5
hours before sunset and 1 hour after sunset.