Advised to ask advice on date S21(4)a

Possession - Section 8 and Section 21

Hi I was going to download form S21(4)a to issue to a tenant who's AST has ended and saw that if the tenancy is weekly I should ask you for advice on dates to put on the form. I issued the AST on 14th September 2011 (which was a Wednesday, though the rent is to be paid on the Friday of each week). The contract term was for 26 weeks, so it expired 26 weeks later on Wednesday 14th March 2012, the tenant having paid his last rent on Friday 9th March and continues to pay rent each Friday to date. The AST has now become a statutory periodic tenancy but I have decided that I will want possesion of the room in 2 months. As rent is paid each Friday, if I want to serve 2 months Notice from tomorrow (Tuesday 20th March 2012), does the 2 full months notice finish on Wednesday 23rd May 2012 or Friday 25th May??