Notice periods
Notice must be in writing, must give the full period required for your tenancy type, and starts from the date it is received, not sent.
Tenant notice
Usually one rental period, in writing. Verbal notice is the most common mistake.
Landlord notice
Longer, and the form matters: the wrong form makes the notice invalid however long it was.
When the clock starts
On receipt. Post adds days — send it in a way you can prove.
Read next: deposit rules, the inventory.