I'm not sure if this has been posted before, but I was looking at the TFL website and found this article.
The first Night Tube services will launch on the Central and Victoria lines on Friday 19 August, with services on the Jubilee, Northern and Piccadilly lines following in autumn. The Night Tube will offer a 24-hour service on Fridays and Saturdays.
www.tfl.gov.uk/campaign/tube-improvements/what-we-are-doing/night-tube?intcmp=22069