It may be asked: why is it Makruh to fast just before Ramadan (for those who do not have a prior habit of fasting)? The answer is that there are a number of reasons why this is so, such as:
1- Lest extra days be added to the fast of Ramadan that are not part of it. Fasting on the day of 'Eid is prohibited for the same reason, lest we fall into the same trap as the People of the Book regarding fasting, as they added to their fasts because of their own whims and desires.
For the same reason it is also forbidden to fast on the “day of doubt”. ‘Ammar, may Allah be pleased with him, said: "Whoever fasts on this day has disobeyed Abu’l-Qasim (blessings and peace of Allah be upon him).
The “day of doubt” is a day when people are not sure whether it is Ramadan or not, when news of the sighting of the crescent moon comes from one whose word cannot be accepted. As for a cloudy day, some of the ‘ulama said that this was also a ‘day of doubt’ and said that fasting was not allowed on this day. This is the view of the majority.
2- To make a distinction between Fardh (obligatory) fasts and Nafl (supererogatory) fasts, because making a clear distinction between Fardh actions and Nafl actions is prescribed in Islam. Hence it is Haram to fast on the day of 'Eid, and the Prophet (blessings and peace of Allah be upon him) forbade following an obligatory prayer immediately with another prayer unless they are separated by saying Salam or speaking, especially in the case of the Sunnah prayer performed just before Fajr.
It is prescribed to make a clear separation between this prayer and the obligatory prayer. Hence it is prescribed to pray it at home and to lie down afterwards. When the Prophet (blessings and peace of Allah be upon him) saw a man praying at the time when the Iqamah had been given for Fajr, he said to him: “Al-Subh is four Rak’ahs.” (Reported by Al-Bukhari, no. 663).
Some ignorant people may think that the reason why we do not fast just before Ramadan is so that we can make the most of eating and have our fill of our desires before we have to deny ourselves by fasting. This is an ignorant mistake on the part of those who think this. And Allaah knows best.