Pradosham falls on the 13th day (Trayodashi) of every lunar fortnight in traditional Hindu calendar. The next Pradosham date is this coming Thursday Feb 27th, 2014. Which also happens to be the Holy Maha Shivarathri Day!
Please see list of Pradsoham dates (IST) for 2014 here:
In the picture, Nandi is worshipped at Arunachalewarar temple on a Pradosham. (picture source: arunachalagrace.blogspot.com)