Praise be to Allaah.
Abu Dawood (3237), al-Tirmidhi (738) and Ibn Naajah (1651) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “When Sha’baan is half over, do not fast.” Classed as saheeh by al-Albaani in Saheeh al-Tirmidhi, 590.
This hadeeth indicates that it is not allowed to fast after halfway through Sha’baan, i.e., starting from the sixteenth day of the month.
But there are reports that indicate that it is permissible to fast at this time. For example:
Al-Bukhaari (1914) and Muslim (1082) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do not anticipate Ramadaan by fasting one or two days before it begins, but if a man habitually fasts, then let him fast.”