Great Northern Rail has announced that from Saturday 14 December 2019 until an undisclosed date in summer 2020 the line between Finsbury Park and Moorgate will be closed on most weekday evenings from 21.40 and on most weekends. On these days trains will run to and from Kings Cross (at a slightly reduced frequency on weekdays).
The closures will allow Network Rail to carry out "intensive maintenance and cleaning of the tunnels" and preparatory work ahead of installing a new signalling system which will increase throughput capacity.
This diagram shows the affected days between December and May (but the closures will continue after May):
Links
Moorgate Line closures (Great Northern Rail website)