There are two types of S21 Notice and it is important that the correct one is issued to the tenant.
Before the expiry of a fixed term Assured Shorthold Tenancy
s21 (1) should be the notice used for the above which gives the tenant two months notice on or by or after a particular date. Landlords are advised not to include an expiry date in the notice and instead state that possession is required after two months from the date of service of the notice.
After the expiry of a fixed term Assured Shorthold Tenancy
s21 (4) can be served on a tenant once the fixed term has come to an end and the tenancy becomes a periodic tenancy. This notice must specify that possession is required after a particular date and that date must be the last day of a period of the tenancy.
If the date specified is not the last day of a period of the tenancy, the notice will be invalid. To combat this landlords are advised to include a savings clause such as "possession to be given on the day in which a complete period of your tenancy expires next after the end of two months from service of this notice"
We know that some district judges may deem a s21 (4) notice invalid if both the expiry date and the savings clause is specified and the dates are different. Their reasoning being that the notice specifies two different dates and s21 (4) requires the notice to specify a date.
Following the case of Spencer v Taylor  EWCA Civ 1600 where in 2006 Ms Taylor rented a property under an AST from Mr Spencer for a fixed term of 6 months. The tenancy continued to run as a periodic tenancy after the expiry of the fixed term. Mr Spencer served a s21 notice on Ms Taylor. The notice included the savings clause but did not provide a specific date which was the last period of the tenancy.
Ms Taylor defended the possession claim on the basis that 1) the notice was invalid as the tenancy was a periodic tenancy and therefore s21(4) applied and 2) that Mr Spencer could not rely on the savings clause.
The Court of Appeal found that the notice was valid because the notice could be interpreted under the following subsection of the Housing Act 1988:
Housing Act 1988 S21 (2) provides:
A notice under paragraph (b) of subsection (1) above may be given before or on the day on which the tenancy comes to an end; and that subsection shall have effect notwithstanding that on the coming to an end of a fixed term tenancy a statutory periodic tenancy arises.
In Spencer v Taylor  EWCA Civ 1600 the Court of Appeal interpreted the above to mean that the notice could be given under this subsection even after the expiry of the fixed term as long as there was a fixed term tenancy in the first place.
Furthermore, despite the inclusion of the savings clause which has in the past been said to create ambiguity, the Court of Appeal found the notice was also valid under s21(4) because Mr Spencer was entitled to rely on the savings clause despite two different dates being specified in the notice. It was confirmed that the date established by the savings clause satisfied the requirement of s21(4) as long as it was clear which date was the "saving" date.
So which notice should you use?
S21 for both fixed term and periodic tenancies with or without a two month expressed expiration date - but always include the savings clause.