Obama will speak at 10:30 am eastern time.
On a State visit, per invitation by the Queen, Obama and Michelle Obama are visiting the UK May 24-26, with a stop--and a speech--first in Ireland. On May 25, Obama will speak at Westminster Hall in London. He will address both houses of Parliament--House of Commons, House of Lords.
The Obamas will stay at Buckingham Palace. That's all the details for now.