For those that intend to keep the Muharram fast, it would be kept on the 9th and 10th, or 10th and 11th of Muharram. Thus, the days to fast would be (tomorrow) Wednesday and Thursday, or Thursday and Friday. To not keep these fasts would not constitute a sin, however, fasting would be greatly rewarding for those that have the ability to do so on these given days in the manner described. May Allah bless each of us to do those actions that would attain us closeness towards Him, Ameen.
Rasulullah (SAW) said: The best fasts after the fasting of Ramadan is that of the month of Muharram.