There are two Pradoshams – one during the waxing phase (Shukla Paksha) of moon and another during the waning phase (Krishna Paksha) of moon. It is said that Shiva and Parvati are in a happy mood during Pradosham period and they shower blessings on devotees.
Pradosham begins around 1.5 hours before sunset and end one hour after sunset. Most Shiva devotees mainly spend this time at a Shiva temple. If at home devotees chant any of the mantras dedicated to Lord Shiva or read the Shiva Purana.
There are staunch Shiva devotees who fast on both the Pradosham. Some people only observe one Pradosham in a month.