Some scholars view that so long as there is sufficient time to make up missed fasts, it is permissible to observe optional fasts.

If you intended to fast `Ashura but you didn’t fast deliberately, you will not get any reward. If you forgot to fast `Ashura, we hope you will get rewarded. If you didn’t fast `Ashura because of sickness, we hope you will get rewarded.

1-The Hadith which reads “Fasting on one of these days (the first ten days of Dhul-Hijjah) is equivalent to fasting for a year” has a da‘if isnad (weak chain of transmission) because of an-Nahhas ibn Qaham and and Mas‘ud ibn Wasil 2- The hadith which reads “There are five things that the Prophet (peace and blessings of Allah be upon him) never failed to do…” is based on the report narrated by Hunaydah ibn Khalid al-Khuza‘i, from whom it was narrated via many isnads, in several versions. The critics of hadith differed as to the classification of this hadith because each of these versions has its own isnad and text. For more, see the detailed answer.

In the case of a voluntary fast that is not done for a specific reason, it is not stipulated that the intention should be formed from the night before. But in the case of a voluntary fast that is done for a specific reason, it is essential to form the intention from the night before.

There is nothing wrong fasting the six days of Shawwal on Mondays and Thursdays, and there will be recorded for you the reward of fasting the six days of Shawwal and of fasting on Mondays and Thursdays.

It is permissible for the one who wants to observe a voluntary fast to form that intention during the day, unlike the obligatory fast, for which it is stipulated that the intention must be formed from the night before.