“Fasting on the 29th of Shaʿban, unless it coincides with a day on which a person habitually fasts.”

Q. I usually fast on Mondays and Thursdays. If the day after the 29th of Sha'ban happens to be one of these two days, is it permissible for me, according to Shari'ah, to fast on that day?

A. The day after the 29th of Sha'ban can either be the 30th of Sha'ban or the 1st of Ramadhan. Due to this uncertainty, the day is called "Yaum-ush-shak" (the Day of Doubt). According to a Hadith in Bukhari, Rasulullah (Sallallaahu 'Alaihi Wasallam) said: "None of you should precede the month of Ramadhan by one or two days of fasting; however, if someone is accustomed to fasting on this day, it will be permissible for him."

As for Yaum-ush-shak, the general rule is not to fast on this day unless it coincides with a day on which the person is accustomed to fasting. In your case, since you usually fast on Mondays and Thursdays, it is permissible for you to fast on Yaum-ush-shak if it happens to fall on one of these two days.

[English version of "Jeuner le 29ème Cha'baan sauf s'il coïncide avec un jour où la personne à l'habitude de jeûner" – Al Haadi – Vol. 3 No. 2]