Correct date of proposed Possession when weekly rent is paid

Possession - Section 8 and Section 21

Hi
I wish to end an room only AST contract at the end of the 6 month period serving a Section 21 notice. to take back the room. The end date of the tenancy is 25th June 2013 having started on 25th January 2013. Due to the fact they pay their rent weekly (Fridays) and the 25th June falls on a Tuesday I am just looking for the correct way to calculate the date to use for the Section 21 notice? Do I calculate a pro-rata rent amount for the Friday prior to the 25th June or is the a better way to do it? Any information greatly appreciated..