Section 8 and Section 21 Dates correct?

Possession - Section 8 and Section 21

Hi, Appreciate some advice with below
1. I served my tenants Section 21 on 8th May to give notice for possession at the end of the current AST (from 06/08/2011 to 05/08/2012), on the Section 21, I put the expiry date as ' After 5th August 2012', but I heard someone says that the Section 21 expiry date should be on the rent date, my rent date is on the 6th every month, Which is one is the correct ? did I get my date right?

2. I also served tenants Section 8 on 7th June 2012 when they are 2 months unpaid, its dated 07/06/2012 on the S8 notice, but was posted on 08/06/2012 (Friday) from 2 post offices with certificates of posting. The expiry date is 26/06/2012(Tuesday), is this date correct? also tenant didn't respond so I made the claim via PCOL on 27/06/2012, is this date right?

3. The hearing will be on 07/08/2012, if there is anything happens against me, can I withdraw Section 8 and start claiming Section 21 straight away,i.e. on 07/08/2012?

thanks