Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
You are permitted to observe fasting in the last day of Shaban so as long as you leave a gap between Shaban and Ramadan. In other words, if you could not fast in the first half of Shaban, you may still fast a few days as long as you do not go all the way to the end of the month.
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
You are permitted to do so as long as you leave a gap between Shaban and Ramadan; in other words, if you could not fast in the first half of Shaban, you may still fast a few days as long as you do not go all the way to the end of the month.
The Prophetic Sunnah was to fast in the first half of Shaban. Aishah, the beloved wife of the Prophet (peace and blessings be upon him) has narrated that the Prophet “used to fast most of the days of Shaban, but he stopped fasting when he got close to the month of Ramadan.” (Al-Bukhari and Muslim)
Based on this, it is considered highly recommended to fast in the early part of Shaban and stop fasting the last few days of the month.
The Prophet wanted to teach us an important wisdom that we should not join Shaban and Ramadan in fasting, rather we should keep the Ramadan fast totally separate from that of Sha`ban. Shaban fasting is only for preparation and warming up for the fasting in Ramadan, so we should not confuse the two fasts.
Almighty Allah knows best.