✅Answer: Please pay attention to the following narrations:

1 عَنْ عَلِيِّ بْنِ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ الْقَاسِمِ بْنِ مُحَمَّدٍ الْجَوْهَرِيِّ عَنْ سُلَيْمَانَ بْنِ دَاوُدَ عَنْ سُفْيَانَ بْنِ عُيَيْنَةَ عَنِ الزُّهْرِيِّ عَنْ عَلِيِّ بْنِ الْحُسَيْنِ فِي حَدِيثٍ طَوِيلٍ قَالَ: وَ صَوْمُ يَوْمِ الشَّكِّ أُمِرْنَا بِهِ وَ نُهِينَا عَنْهُ أُمِرْنَا بِهِ أَنْ نَصُومَهُ مَعَ صِيَامِ شَعْبَانَ- وَ نُهِينَا عَنْهُ أَنْ يَنْفَرِدَ الرَّجُلُ بِصِيَامِهِ فِي الْيَوْمِ الَّذِي يَشُكُّ فِيهِ النَّاسُ فَقُلْتُ لَهُ جُعِلْتُ فِدَاكَ فَإِنْ لَمْ يَكُنْ صَامَ مِنْ شَعْبَانَ شَيْئاً كَيْفَ يَصْنَعُ قَالَ يَنْوِي لَيْلَةَ الشَّكِّ أَنَّهُ صَائِمٌ مِنْ شَعْبَانَ- فَإِنْ كَانَ مِنْ شَهْرِ رَمَضَانَ أَجْزَأَ عَنْهُ وَ إِنْ كَانَ مِنْ شَعْبَانَ لَمْ يَضُرَّهُ فَقُلْتُ وَ كَيْفَ يُجْزِي صَوْمُ تَطَوُّعٍ عَنْ فَرِيضَةٍ فَقَالَ لَوْ أَنَّ رَجُلًا صَامَ يَوْماً مِنْ شَهْرِ رَمَضَانَ تَطَوُّعاً وَ هُوَ لَا يَعْلَمُ أَنَّهُ مِنْ شَهْرِ رَمَضَانَ- ثُمَّ عَلِمَ بِذَلِكَ لَأَجْزَأَ عَنْهُ لِأَنَّ الْفَرْضَ إِنَّمَا وَقَعَ عَلَى الْيَوْمِ بِعَيْنِهِ.

📚 Al-Kafi, vol. 4, p. 85

Imam Sajjad, peace be upon him, said in a narration: 'We have been both commanded and forbidden regarding the fast of the day of doubt. We have been commanded to fast it along with the fast of the month of Sha'ban, and we have been forbidden from it in the way that a person wants to fast it individually on a day that people are in doubt about (as part of the month of Ramadan).'

I said: 'May I be your sacrifice, what should a person who has not fasted anything from the month of Sha'ban do?' He said: 'On the night of doubt, he should make the intention to fast it as part of the month of Sha'ban. So if it is from the month of Ramadan, it will suffice for him, and if it is the month of Sha'ban, there is no harm for him.'

I said: 'May I be your sacrifice; how can a supererogatory fast suffice for an obligatory fast?' He said: 'If a person fasts a day from the month of Ramadan as a supererogatory fast while he does not know that day is from the month of Ramadan, and then he finds out that it was Ramadan, it will suffice for him because the obligatory duty has occurred on it exactly.'

2 عَنْ مُحَمَّدِ بْنِ أَبِي الصُّهْبَانِ عَنْ مُحَمَّدِ بْنِ بَكْرِ بْنِ جَنَاحٍ عَنْ عَلِيِّ بْنِ شَجَرَةَ عَنْ بَشِيرٍ النَّبَّالِ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: سَأَلْتُهُ عَنْ صَوْمِ يَوْمِ الشَّكِّ فَقَالَ صُمْهُ فَإِنْ يَكُ مِنْ شَعْبَانَ كَانَ تَطَوُّعاً وَ إِنْ يَكُ مِنْ شَهْرِ رَمَضَانَ فَيَوْمٌ وُفِّقْتَ لَهُ.

📚 Al-Kafi, vol. 4, p. 82

Bashir al-Nabbal said: 'I asked Imam Sadegh, peace be upon him, about the day of doubt.' The Imam said: 'Fast it, so if it is part of the month of Sha'ban, the fast will be considered a supererogatory and voluntary one, and if it is from the month of Ramadan, it is a day you have been successful in observing.'

✅In all the narrations about the day of doubt where the type of fast is specified, a supererogatory fast has been designated for it. And no narration has been seen that designates or permits a non-supererogatory fast for the day of doubt. Therefore, if the 30th of Sha'ban is the day of doubt, we should only observe a supererogatory fast on it and with the intention of it being part of the month of Sha'ban, even if a person has a missed fast on their account, so that, if Allah wills, we meet our Imam and gain complete knowledge about this matter, if Allah wills.

✅Also, pay attention to the following narration:

3 عَنْ مُحَمَّدِ بْنِ أَبِي عُمَيْرٍ عَنْ حَفْصِ بْنِ الْبَخْتَرِيِّ وَ غَيْرِهِ عَنْ عَبْدِ الْكَرِيمِ بْنِ عَمْرٍو قَالَ: قُلْتُ لِأَبِي عَبْدِ اللَّهِ إِنِّي جَعَلْتُ عَلَى نَفْسِي أَنْ أَصُومَ حَتَّى يَقُومَ الْقَائِمُ فَقَالَ صُمْ وَ لَا تَصُمْ فِي السَّفَرِ وَ لَا الْعِيدَيْنِ وَ لَا أَيَّامِ التَّشْرِيقِ وَ لَا الْيَوْمِ الَّذِي يُشَكُّ فِيهِ.

📚 Al-Tahdhib, vol. 4, p. 183

Abdul Karim said: 'I said to Imam Sadegh, peace be upon him: 'I have decided for myself (vowed) to fast until your Riser (the Qa'im) rises!' The Imam said: 'Fast! (There is no problem), but do not fast during travel, on Eid al-Fitr, Eid al-Adha, the Days of Tashriq (i.e., the 11th, 12th, and 13th of Dhu al-Hijjah), and on the day of doubt!'

✅In the above narration, it is also observed that a vow fast, which is one of the obligatory fasts, is not permitted to be observed on the day of doubt.

اللَّهُمَّ صَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ

وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ