Is it allowed to fast the day before Ramadan as a precaution (the "day of doubt")?

No. The "day of doubt" (yawm al-shakk) is the 30th of Sha'ban when the crescent could not be sighted on the 29th night because of cloud or haze. The Prophet ﷺ said: "Do not anticipate Ramadan by fasting a day or two before it, except a man who was already fasting his usual fast — let him fast that day." So fasting that day "just in case" it is Ramadan is not allowed; the month is entered only by a confirmed sighting of the crescent or by completing thirty days of Sha'ban. The exception in the hadith is for someone with an established habit that happens to fall on that day — e.g. his regular Mondays and Thursdays, a make-up fast, or a vow. 'Ammar ibn Yasir رضي الله عنه said: "Whoever fasts the day about which there is doubt has disobeyed Abu al-Qasim ﷺ" — narrated in the Sunan collections and quoted by al-Bukhari as a chapter heading in his Book of Fasting. If confirmed news of the sighting reaches people during the day, they stop eating out of respect for the month and make that day up afterwards.

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