There are certain days in the year on which fasting will entail greater rewards, but as long as a person has due obligatory fasts, such as qada fasts on them, they cannot observe recommended fasts. However, if one forgets that they have some obligatory fast to perform and fasts are recommended fast, then they remember this before noon they can change their Niyyah to that of an obligation obligatory fast. But if they remember that they have obligatory fast to perform in the afternoon, their fast will be invalid. As for Yawm al-shak, it has rules of its own. Yawm al-shak is a day about which one is doubtful whether it is the last day of Sha’ban or the first day of the month of Ramadan, so it is not obligatory to fast on it.
If one intends to fast on this day, they can do it with the Niyyah of a recommended fast or a due obligatory fast. Then, if later it is proven that it has been the first day of the month of Ramadan, their fast will be regarded as the fast of the month of Ramadan. However, if one becomes certain during that day that it is the first day of the month of Ramadan, they must immediately change their Niyyah in to the first day of Ramadan. A few necessary points. One, if an immature child becomes mature before the fajr time in the month of Ramadan, they must fast on that day.
However, if they become mature after the fajr time and they have not done any of the invalidators of the fast, they, as an obligatory precaution, must make their Niyyah for fasting and fast on that day, they must also perform later the qada of that fast as well. Likewise, if a sick person who has been exempt from fasting fully recovers before the noon and they have not done anything that would invalidate a fast, they must, as an obligatory precaution, make their Niyyah for fasting and fast on that day. They must also perform the qada of that fast later. But if they get well in the afternoon, fasting on that day will not be obligatory upon them, and they only have to perform the qada fast of that day. A disbeliever who converts to Islam before the noon adhān in the month of Ramadan who has not done anything that would invalidate the fast, must, according to obligatory precaution, continue fasting on that day.
However, they do not need to perform the qada fast later.