How the date is fixed
Trayodashi, the thirteenth day of each half, kept on the day it is running at pradosh, the twilight after sunset. It is named for its weekday: Soma Pradosh, Shani Pradosh and so on.
About
Pradosh vrat is kept twice a month, on the Trayodashi of each fortnight, during the pradosh period of roughly 96 minutes after sunset. The day is chosen so that Trayodashi is running at sunset, even if it is absent at sunrise; when it touches sunset on two days, the first is usually taken. Each weekday gives the observance a name, Soma Pradosh on Monday and Shani Pradosh on Saturday being the ones most often singled out. In Tamil Nadu the Pradosham puja at Shiva temples, with the Nandi abhisheka, is a regular feature of temple calendars.
How it is observed
A fast until dusk and Shiva worship with abhisheka in the pradosh window after sunset.