Question 8: I am a young man whom Allaah (Exalted be He) guided to the right path. I like to observe Sawm (Fast) on the Day of ‘Ashooraa’ (10th of Muharram) and on all other virtuous days beside Ramadaan.
My question is whether it is permissible to depend on the calendar to determine the start of Muharram, or is it better to observe Sawm on the two days preceding and succeeding ‘Ashooraa’ to be on the safe side? May Allaah reward you with the best!
Answer: You have to depend on sighting the new moon, but if the efforts to sight the new moon failed, stay on the safer side by completing Dul-Hijjah thirty days... read more here.