clarification needed S21

Possession - Section 8 and Section 21

Hi
I need some clarification as to whether I have issued a section 21 correctly before I continue through the court system.
6 Month tenancy. From 17 feb 2012 to 16 Aug 2012.
Realized that tenants were not good for long term as causing nuisance to neighbors. Issued a S21 during the AST term (25th June) BUT 2 months takes it to 26 Aug which is 10 days outside of the AST.
the rent frequency on AST was set as per calendar month. Rent received was direct from Housing benefits as LHA rate on a 4 weekly basis, which didn't cover whole of rent. No top-up was ever received.

I set the date for s21 as 26 Aug was this correct?