z-dayhttp://www.oed.com/dictionary/z-day_n%3Ftab%3Dmeaning_and_useZ-day, n.
First published 1986; not fully revised

Z-daynoun

Military.
  1. 1925–
    1. 1925
      The opening of the main attack of the battle of the Somme was fixed for June 29th, and notified beforehand as ‘Z Day’.
      E. Fraser & J. Gibbons, Soldier & Sailor Words 312
    2. 1930
      Operation Orders..notified us that Thursday was ‘Z’ (or zero) day.
      S. Sassoon, Memoirs of Infantry Officer iv. 61
    3. 1938
      In the sour concrete hole the corporal shows his muddy map, his Z Day zone of fire.
      E. Blunden, On Several Occasions

Originally published as part of the entry for Z, n.

Z, n. was first published in 1921; not fully revised.