In the case of Spencer v Taylor  EWCA Civ 1600, S granted an Assured Shorthold Tenancy to T on 6 February 2006, (a Monday) for a fixed terms of 6 months with rent payable weekly.
This meant that the first day of each period was a Monday and the last day was a Sunday. At the end of the fixed term a statutory periodic tenancy arose running from a Monday to a Sunday.
In October 2011 a s21 notice was served giving an expiry date of 1 January 2012 (a Saturday) and also including, as an alternative, the usual saving provision: "Or (b) at the end of your period of tenancy which will end next after the expiration of two months from the service upon you of this notice."
Until this decision, the accepted view had been that (1) if the notice was served before or on the day on which a fixed term tenancy ends then a two month notice was all that was required, but (2) if the notice was served after the fixed term had ended (during a statutory periodic tenancy), then the notice must give two months and must expire at the end of a relevant period.
The position is now that if a tenancy has at some stage had a fixed term then a notice which gives two months notice is valid, whether it is being served during a fixed or periodic part of the tenancy. A notice which expires at the end of a period of a tenancy is only required for tenancies that were periodic from the outset and have always been so.
The majority of tenancies in England & Wales have at some stage been operating under a fixed term and so a notice giving two months notice will suffice for them.