Is it permissible to fast to make up a vowed fast during the last days of Sha'ban, or must it be postponed until after Ramadan?

The conclusion is that there is no harm in fasting a vowed day in the second half of Sha'ban, because the most correct opinion is that fasting therein is not disliked. And those who hold it to be disliked make an exception for habitual fasting, and for obligatory fasts such as making up missed fasts (qada'), vows (nadhr), and expiations (kaffarah).