Is it valid to fast the ninth day of Muharram with the intention of fasting the day of Ashura, if one does not come to know about the day of Ashura until that very day?

The majority of scholars hold that the Day of Ashura is the tenth of Muharram, while Ibn Abbas was of the opinion that it is the ninth. Based on the view of the majority, if you did not observe the Day of Ashura due to a calendar error, but you were keen to fast it, then you will hopefully attain its reward. The reward is written for whoever intended a good deed but was prevented from it by an unavoidable obstacle, as stated by Sheikh Ibn Uthaymeen. A woman who was prevented from fasting the tenth due to menstruation does not have to make it up, as it is a Sunnah tied to its specific time.