The soldier was shot while on patrol in Helmand Province. She was the third British woman to have died while serving in Afghanistan since 2001.
The service is expected to begin around 12:00 GMT.
Cpl Day, 25, of 3 Medical Regiment and Royal Marine Cpl David O'Connor were fatally injured in a gun battle. The soldiers were in a firefight while on patrol. An Afghan man, thought to be an off-duty policeman, also died.
Cpl Day joined the Army in 2005 and was deployed to Afghanistan on 2 October.