Hi Fiona177, thanks for posting and welcome to our community.
As @Anonymous has already advised any notice period would commence 30 days from the date officially given. When I say officially given, it would have be done securely. In this case you would be able to call on today to give your notice, but you would need to advise that you wish your services to end on 15th May. As my colleagues in Customer Relations can accept anywhere up to 60 day notice.