However, if the tenant has a 'fixed term' tenancy (i.e. one which specifies a termination date), you can
serve a Section 21notice at any time but the
expiry date must be:
(a) at or beyond the scheduled date for the termination of the tenancy ;
(b) at least two months after the date on which the notice is served ; and
(c) at least 6 months after the commencement of the original tenancy agreement.
In certain circumstances (where the tenant has broken the tenancy agreement, e.g. by not paying rent), some, or all, of those conditions might not apply. (If a tenant decided to 'sit tight', a court would automatically grant an eviction order as long as those three conditions were met. Otherwise, you'd have to show that the relevant special circumstances applied).
For an authoritative statement, see Section 6 of this document:
http://www.communities.gov.uk/pub/899/Assureda ndassuredshortholdtenanciesAguideforlandlords_ id1151899.pdf
Chris