As salamo alaikum,
Imam Jafer Sadiq(a.s.) reports that Prophet Mohammad(sawaw) has said that Shabaan is my month and Ramzan is the month of Allah(swt). Therefore, if anyone observers one day of fasting during Shabaan, I will do his/her "shafaat" on the day of judgment, and one who observers two days of fasting then his/her previous sins would be forgiven and if someone observers three days of fasting then it would be just like all the sins have been washed away.
Imam Ali Riza(a.s.) has said that if someone observes fast during the last three days of Shabaan and combines them with the fasting during the month of Ramazan then Allah(swt) will grant the sawab of two months of continuous fasting to that person.
It is, therefore, advisable to fast in the last three days of Shabaan to become eligible for a great Sawab
This year Ramzan is predicted to start on either the 1st September on some places or the 2nd of September at others. Therefore it is advisable to start fasting from Firday, the 29th of August 2008.