The JR Yamanote Line will be partially closed on April 19th (Saturday) and the 20th (Sunday). The right half of the circular line that connects important areas of Tokyo will be affected the most.
Be careful if you were planning to use the Yamanote Line to connect to a Shinkansen on these dates.
Access to Hamamatsucho Station (Connecting to Tokyo Monorail for Haneda Airport) will be heavily impacted. For access to/from Haneda, consider the Airport Limousine Bus or Keikyu Train to minimize loss of time.)
April 19
All day: there will be no service between Ueno-Tokyo-Shinagawa-Ohsaki section for trains going clockwise.
From first train to 5AM: No service for the Ohsaki-Shinagawa-Tokyo-Ueno section going counter-clockwise.
Reduced services for the rest of the day.
Remaining section will operate at a reduced frequency.
April 20
First train to 12 Noon: No service between Ueno-Tokyo-Shinagawa-Ohsaki section for trains going in both directions.
Remaining sections will run at a reduced frequency through noon, then all sections will resume normal operation in the afternoon.
The Keihin Tohoku Line, which runs parallel to the Yamonote Line will also suspend service between Shinagawa and Higashi-Jujo all day on the 19th and until noon on the 20th. The Ueno-Tokyo Line runs normally but stops are only made at Ueno, Tokyo, Shimbashi and Shinagawa.
The suspension is to allow for work of a new JR line that will eventually connect Tokyo Station and Haneda Airport directly. The link is scheduled to open in 2031.
Official announcement on the JR East homepage: https://www.jreast.co.jp/tamachi-koji/