Yesterday (26th Oct 2010) was a special Sankashta Chaturthi as it fell on a Tuesday. I heard that fasting on this day is equivalent to observing the fast for 21 days :). Researching on internet, found this link that mentions it to be Angarak Ganesh Chaturthi and explains why it’s special. Thought it’s a good time to start fasting on this day – with only fluids until moonrise. But reading through this link, it looks like you cannot have any fluids except water and that too in limited quantities.
Dates of sankashta chaturthi for this year are here.