Yawm al-Shakk (Arabic: "یوم الشک") which literally means the "doubtful day" is a term commonly used in Fiqh and refers to two days in the Lunar Hijri Calendar that are not considered from either previous or next month, till the crescent of the new moon is certainly seen by the people; The day after Sha'ban 29 and the day after Ramadan 29 of every year are two samples of Yawm al-Shak. At the evening of Sha'ban 29, many people go out for moon sighting. If they see the new moon's crescent, the next day would be announced as the first day of Ramadan, so everyone has to fast. The same thing goes for the day after Ramadan 29, which could be either the 30th day of Ramadan or the 1st day of Shawwal and Eid al-Fitr in which the fasting is forbidden in Islam.
Origin and Importance
Every month of the Lunar Calendar is either 29 or 30 days, which starts when the new moon's crescent first becomes visible and ends when the same thing happens 29 days later or when 30 days passed because a lunar month could never be more than 30 days.
Though the beginning and end of every lunar month are certainly known and calculated by the scientific rulings of Astronomy, the visibility of the crescent is the criteria for the beginning of every lunar month. The beginning of Ramadan and Shawwal is more important for Muslims because of this Islamic ruling that considers the new moon's visibility for the eyes of people, as the criteria for the beginning and end of Ramadan. In other words, Muslims have to start fasting on the day after they see the crescent of the new moon of Ramadan by their eyes and to stop fasting when they see the new moon of Shawwal the same way.
The importance of the beginning and the ending of Ramadan and the recommendation of Istihlal (moon sighting) is since fasting on this day is either Wajib (obligatory) or Haram (forbidden). If the day after Ramadan 29 is proven to be the 30th day of Ramadan, fasting will remain obligatory, but if the new moon is seen, the next day would be Eid al-Fitr, and it is forbidden for Muslims to fast is this Eid.
Beginning of Ramadan
According to the Fiqh principles, the doubtful day, belongs to the previous month (Sha'ban), and fasting in it is not obligatory, just like other days of Sha'ban; but it can be fasted in the day with the intention of recommended or compensation fast, but fasting with the intention of obligatory fasting of Ramadan is not permitted. If a person fasted in the day with the aforementioned intentions and later he realized that it was the first day of Ramadan, his fasting is automatically considered as the obligatory fasting of Ramadan. If later he realized that the day was the last day of Sha’ban, the fasting is considered as he has intended. If someone didn’t fast in the doubtful day and in the middle of the day he realized that it is the first day of Ramadan, if he hadn’t done any of the fasting breakers, it’s obligatory for him to resume his fasting with the intention of obligatory fasting and his fasting is correct. Still, if he had done one of the fasting breakers (for example he had eaten something) he cannot fast in the day, but he has to avoid fasting breakers for the rest of the day, and compensate for the fasting in the day after the Ramadan.
The End of Ramadan
According to the same principle mentioned above, the doubtful day belongs to the previous month (Ramadan), so the day which we doubt about it being the 30th of Ramadan or the 1st of Shawwal, is considered the last day of Ramadan and its fasting is obligatory. If in the middle of the day it is realized that the moon has been seen in the last night it’s obligatory to break the fast because fasting in the 1st of Shawwal (Eid al-Fitr) is forbidden.
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