British intelligence expected Germany to attack Crete between about 15 and 20 May 1941, and the airborne landings began on 20 May. Intelligence estimates normally use windows rather than exact dates. Naming a precise day would imply unusually strong information, while a range represents the underlying uncertainty more honestly. This is the level of warning an operational commander can reasonably expect before an attack.
Notable Point · 7:02-7:34
The Crete attack window shows how intelligence estimates work
The forecast brackets the German assault within mid-May and explains why military intelligence usually gives a window rather than an exact day.