Sentence:  This morning I got up, with great difficulty, at 6.30, and at 7.45 we started out for a Brigade <WSD>Field Day</WSD>. Did an attack from 10.30 to 2.30, but it wasn't a strenuous one for me as I was told to "become a casualty" soon after the 3000 yard assault began …. 
Meaning of the word marked with <WSD> tags:  A day for manoeuvres and tactical exercises in "the field".