Question: In regards to the month of Muharram this year, the ninth will be on a Friday and the tenth on Saturday. Is it allowed to fast Friday and Saturday?
Answer: “Yes, because he fasts a day with it. There is no harm”
Question: If I fast Thursday, three days?
Answer: “All of it is the same. Fast Thursday, Friday, Saturday. These three days is better. If he fasts one day before or after, then there is no harm”
Faisal Ibn Abdul Qaadir Ibn Hassan