A: Nabi (Sallalahu Alaihi Wasallam) used to fast excessively during the month of Sha'ban, more than any other month. Hence, one may fast as much as one wishes in the first half of Sha'ban. With regards to fasting on the 13th, 14th and 15th or only the 15th, then one may fast on 13th, 14th and 15th, or only on the 15th. Please note that these are optional fasts.
And Allah Knows Best
Darul Ihsan Fatwa Department