The day of fasting for Pradosham can differ for two cities even if the cities are within the same state. Fasting for Pradosham depends on the sunset time and it is observed when Trayodashi Tithi prevails after sunset. Hence Pradosham fasting might be observed on Dwadashi Tithi i.e. one day before Trayodashi Tithi. As Sunset time is different for all cities, it is important to refer Hindu calendar like this website which lists location based Pradosham days. As generating location based dates are time consuming most sources ignore this fact and publish single list for all cities.
Although Karthigai day is observed every month but the most significant day falls in month of Karthikai (which is same as Solar month Vrischika in other Hindu calendars). DrikPanchang.com lists all Karthikai days in the year including the most significant Karthikai day which is known as Karthikai Deepam.Karthigai festival in Tiruvannamalai hills is very famous. On Karthigai day, a huge fire lamp is lit up on the hill, visible for several kilometers around the hill. The fire or Deepam is called Mahadeepam, Hindu devotees visit here and pray to God Shiva.
There are twelve Rohini fasting days in a year. Usually Rohini Vrat is observed for three, five or seven years continuously. The advisable duration of Rohini fasting is five years and five months. Rohini fasting should be finished with Udyapana.